THOUSAND OAKS, Calif., Feb. 7— Jack Kirby, an artist who helped create the popular comic-book characters Captain America and the Incredible Hulk -- superheroes with human characteristics, for a new generation of readers -- died on Sunday at his home here. He was 76.

The cause was heart failure, said his son, Neal.

Mr. Kirby was one of the artists credited with reinventing superheroes in the late 1950's and early 60's, by portraying them as more human and even vulnerable. Under his influence, comic-book story lines, which had traditionally been short, were expanded, leading to the issue-length format.

"Kirby is to comics what Louis Armstrong is to jazz," said Greg Theakston, who publishes comic books through his New York-based company Pure Imagination. "They were both there at the birth of a new art form and strongly influenced it, even defined what the form was." Born in New York

Mr. Kirby, whose original name was Jacob Kurtzberg, was born in New York City in 1917. He began work in the comics industry in the 1930's. By 1940, he had teamed up with Joe Simon, his partner for the next 15 years. In 1940, they created Captain America, and in 1942 they produced the "Boy Commandos" comics. That story, about young soldiers, sold a million copies per issue, Mr. Theakston said.

After World War II, Mr. Kirby and Mr. Simon teamed again to create popular-romance comics. "Jack's style was dynamic," Mr. Simon said. "He brought the action drawing to a new level. His style was imitated all over and still is today to a certain extent."

In 1958, Mr. Kirby went to work for Marvel Comics, where he collaborated with the dialogue writer and editor Stan Lee. They created such characters as Mighty Thor and the Incredible Hulk.

In the early 1970's, he began work on a long series for DC Comics and drew for "Mister Miracle," "New Gods" and "Jimmy Olsen."

After another stint with Marvel, he helped design animated films, including "Thundarr the Barbarian" and the "Fantastic Four" television series. His last full comic book appeared in 1986.

In addition to his son, he is survived by his wife, Rosalind; three daughters, Barbara, Lisa and Susan, and two grandchildren.

Photo: Jack Kirby (Bill Bridges, 1971) and one of his creations, Captain America. ($; Marvel Comics Group)