Diana Hyland, costar of the new television series, "Eight Is Enough," died Sunday at her home in the Westwood section of Los Angeles. The cause of death has not been determined.

Miss Hyland, who was in her late 30s, played the role of Joan Braden, who with her husband, Tom, copes with the problems of rearing eight children.

The series, broadcast on ABC-TV, is based on a book written by columnist John Travolta in ABC's "The boy in Tom Braden.

Considered a glamorous character actress with versatility, Miss Hyland recently had played the mother of the Plastic Bubble."

She also had appeared on "Happy Days," as the girl friend of The Fonz.

Born in Cleveland Heights, Ohio, she was discovered at the age of 15 when her picture appeared in a Cleveland newspaper. On Broadway, she debuted in "Look Back in Anger" and played in "Sweet Bird of Youth."

On television, Miss Hyland had played the wife in "Yound Doctor Malone," the alcoholic Susan Winter in "Peyton Place" and won an Emmy nomination for her performances in "The Voice of Charlie Pont."

She was a guest star on many television shows, including "Kojak," "The Fugitive." "Harry O," "Barnaby Jones" and "Dr. Kildare." She made her film debut as Mrs. Norman Vincent Peale in "One Man's Way."

She is survived by a son, Zachary, and her parents, Mr. and Mrs. John Genter, of Cleveland Heights.