H. R. Haldeman, who spent 18 months in prison for his involvement in the Watergate scandal but said he believed his role as President Richard M. Nixon's chief of staff was the "mountaintop experience" of his life, died yesterday at his home in Santa Barbara, Calif. He was 67.

A son, Hank, said that the cause was an abdominal tumor and that his father had been ill for a month.

Mr. Haldeman resigned as the White House chief of staff on April 30, 1973, and was convicted two years later on charges of perjury, conspiracy and obstruction of justice in trying to cover up the Nixon Administration's involvement in the Watergate scandal. He was sentenced to two and a half to eight years in prison, but the sentence was later reduced to the 18 months that he served at the minimum security Federal prison in Lompoc, Calif.

Since December 1978, when he was released from prison, Mr. Haldeman worked as a businessman and business consultant. He had some real estate interests and held the franchises on eight Sizzler Family Steak Houses in Florida. Began With a Break-In

Watergate, one of the most far-reaching political scandals in American history, began on June 17, 1972, when five men who worked for the Committee to Re-elect the President were arrested in a predawn break-in while trying to plant electronic eavesdropping devices at the Democratic National Committee's headquarters in the Watergate office and housing complex in Washington.

Information that was developed in several investigations led to impeachment proceedings against Mr. Nixon, who resigned from the Presidency on Aug. 9, 1974, still protesting his innocence.

Although Mr. Haldeman said that his admiration for Mr. Nixon's abilities as a statesman never diminished, he asserted in his 1978 book, "The Ends of Power," that Mr. Nixon had "initiated the Watergate break-in" and participated in its cover-up from "Day 1."

In the national anguish over Watergate that grew in the aftermath of the disclosures, Mr. Haldeman was frequently depicted as deeply involved in the Administration's conduct of political espionage, which it carried out against those regarded as actual or potential enemies of the President. Assessing His Colleagues

But Mr. Haldeman insisted, both in his book and in public statements, that he had little interest in such things and really knew relatively little about them, compared with other Nixon aides like Charles W. Colson and Mr. Haldeman's longtime college friend, John D. Ehrlichman. Mr. Colson, the special counsel to the President, "encouraged the dark impulses in Nixon's mind," Mr. Haldeman said. And he said that Mr. Ehrlichman, the President's chief adviser on domestic matters, had "the more devious approach."

Mr. Haldeman said that when he first learned of the Watergate break-in, he recalled smiling and asking: "Wiretap the Democratic National Committee? For what?"

To explain why so many people regarded him as a principal in the Watergate scandal, he said "the press has created the image of me as a monster." His own inclination was not to be sneaky, he said, but to be direct and above-board with people. But he acknowledged that there were times when he seemed "brusque" when he had "chewed people out."

Mr. Haldeman's brusqueness was both corroborated and widely reported in the press. In his heyday, much was made of his crew-cut coiffure and imperious ways, which were likened to the techniques used by Marine Corps drill instructors. In various published descriptions of him, he was said to have "a gaze that would freeze Medusa" and the bearing of a "Prussian guard."

As Watergate unfolded, it appeared that Mr. Haldeman's activities exceeded those ordinarily assigned to a man of his station, and not all were accounted for in Watergate.

For example, Dan T. Carter, a professor of history at Emory University and an expert on the Nixon years, contended in an article on the Op-Ed page of The New York Times in 1988 that Mr. Haldeman had authorized secret cash payments of $400,000 to strengthen political opponents of George C. Wallace in the 1970 race for governor of Alabama.

And when Mr. Wallace bested those opponents, Mr. Haldeman, with Mr. Nixon's approval, "promoted a Federal tax probe of Mr. Wallace" and disclosed preliminary results of the investigation to the syndicated columnist Jack Anderson, Professor Carter said. His contentions went unanswered by Mr. Haldeman.

In addition, in 1987, a California researcher, Robert Ranftel, obtained a memorandum of a conversation that Mr. Haldeman had in 1970 with J. Edgar Hoover, then Director of the Federal Bureau of Investigation. The memo, obtained through a Freedom of Information Act request, indicated that Mr. Haldeman had asked Mr. Hoover to provide any information he had on "known and suspected" homosexuals in the Washington press corps. Mr. Haldeman later said he could not recall talking about this with Mr. Hoover. A Well-to-Do Family

Harry Robbins Haldeman, who liked to be called "Bob," was born on Oct. 27, 1926, in Los Angeles and reared in a well-to-do neighborhood of Beverly Hills. His father, Harry Francis Haldeman, was a successful businessman, first in plumbing and later in air-conditioning. His grandfather, Harry Marston Haldeman, was a co-founder of the Better American Federation of California, one of the nation's first anti-Communist organizations.

But young Bob Haldeman said he was not brought up in an intensely political household and that he cared little about politics when he attended the University of California at Los Angeles, from which he was graduated in 1948.

"I was not only apolitical," he wrote, "but had gone completely the other route. I was a rah-rah college type, a homecoming chairman, no less, and a campus leader."

While in college, he found a kindred spirit in John Ehrlichman, whom he later invited to work for Mr. Nixon. After college Mr. Haldeman joined the J. Walter Thompson advertising agency on the West Coast, rising through the ranks to supervise accounts like those of Disneyland and Sani-Flush. Hired Future Nixon Aides

Among those he hired while in the advertising business were two who later served in the Nixon White House: Ron Ziegler, who, as the Presidential press secretary, once described the Watergate break-in as "a third-rate burglary attempt," and Dwight Chapin, who became the President's appointments secretary.

Mr. Haldeman first worked for Mr. Nixon in 1956, when he ran for re-election as Vice President on the Republican ticket headed by President Dwight D. Eisenhower. Mr. Haldeman later wrote that he had been attracted to Mr. Nixon because he admired his "fighting" spirit.

The Nixon forces were so impressed with Mr. Haldeman that in 1960, when Mr. Nixon ran for President against John F. Kennedy, Mr. Haldeman was induced to take another leave from J. Walter Thompson to serve as chief advance man.

Besides his son Hank, Mr. Haldeman is survived by his wife, the former Joanne Horton, another son, Peter, and two daughters, Susan and Ann.