Harvey Martin, the Dallas Cowboys star defensive end who was co-winner of the most valuable player award in the 1978 Super Bowl, died Monday at Baylor Medical Center in Grapevine, Tex. He was 51.

The cause was pancreatic cancer, Maureen Porter, a spokeswoman for the medical center, said.

Martin, who played 11 seasons in the National Football League, all with the Cowboys, was a premier pass-rushing end. He holds team records for career sacks, with 113, and single-season sacks, with 20, achieved in 1977, when he was the N.F.L. defensive player of the year. A speedy 6 feet 5 inches and 250 pounds, Martin led the Cowboys in sacks seven times. He was named to the Pro Bowl from 1976 through 1979.

At right end, Martin was part of a fearsome defensive line that included Randy White at right tackle and Ed (Too Tall) Jones at left end when the glamorous Cowboys were known as America's Team.

Martin's greatest day came in the Cowboys' 27-10 victory over the Denver Broncos in Super Bowl XII at New Orleans on Jan. 15, 1978. He had two sacks and deflected a pass, sharing M.V.P. honors with White. Their pressure helped induce Craig Morton, the Broncos' quarterback, to throw four interceptions.

Martin was one of the Cowboys' most popular players. In 1977, he was host of ''The Beautiful Harvey Martin Show'' on a Dallas radio station, owned two restaurants, appeared at charity events and worked during the off-season as a television sports reporter.

''There was this billboard that Oak Farms Dairy had on Central Expressway, with famous people drinking milk,'' Dwight White, Martin's teammate at East Texas State University and later a Pittsburgh Steelers star, once recalled. ''I looked up, and there was Harvey. He was as big as you could get.''

But the good times were shadowed by suspicions of drug use.

Martin was questioned in 1982 by the Federal Bureau of Investigation in a case involving a defendant later convicted of selling cocaine. In the summer of 1983, federal law enforcement officials said that Martin and several other Cowboys players had been mentioned in an F.B.I. wiretap as drug users. Martin denied ever using cocaine and no criminal charges were filed against him.

In August 1996, Martin was arrested on cocaine and domestic-violence charges after an argument with a girlfriend at a Dallas apartment. He pleaded no contest to a felony cocaine possession charge and was ordered into a drug treatment center.

Martin had acting roles and appeared at memorabilia shows after leaving football.

He is survived by a son, Devincent Robertson Martin; a daughter, Chase Martin; his parents, Sylvester and Helen Martin, and a sister, Mary.

A native of Dallas, Martin played at East Texas State (now Texas A&M University-Commerce) after only one year of high school football, then was a third-round draft choice of the Cowboys in 1973.

Two members of the Steelers' famed Steel Curtain defensive line -- Dwight White and Joe Greene, who had played at North Texas State -- took Martin to a park in Grand Prairie, Tex., to teach him techniques.

''Tricks like how to make the move inside after faking to the outside, how to turn the offensive lineman so his own momentum carries him past you,'' Martin remembered. ''After about two months, I went to camp with a little edge on other rookies.''

But he evidently lacked toughness.

The former Cowboys wide receiver Drew Pearson recalled that Martin was in danger of being cut as a rookie, but altered his disposition after meeting with the defensive coordinator, Ernie Stautner.

''Ernie told him, 'You're too nice,' '' Pearson remembered. ''Harvey changed overnight. All of a sudden, Too Nice became Too Mean.''

In the Cowboys' 1977 media guide, Martin described his pass-rush technique: ''Combine speed with 250 pounds and squeeze.''

Photo: Harvey Martin was co-most valuable player of Super Bowl XII in 1978.

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