Actress Felicia Montealegre Bernstein, 56, the wife of composer-conductor Leonard Bernstein, died yesterday at their Long Island, N.Y. home. She had been ill with cancer for several years.

Mrs. Bernstein was born in Costa Rica. She was educated in Santiago, Chile. The Bernsteins, who were married on Sept. 9, 1951, had three children: two daughters, Jaime, 24, and Nina, 16, and a son, Alexander, 22.

As the wife of one of the world's most in-demand musicians, Mrs. Bernstein had little time to follow a stage career, although she appeared in new plays on Broadway from time to time.

In [WORD ILLEGIBLE] she had been named television actress of the year for her leading roles in Studio One, Kraft Theater and Philco Theater.

When her husband's Third Symphony, called "Kaddish," was given its U.S. premiere by the Boston Symphony Orchestra under Charles Munch in January 1964, Mrs. Bernstein was the narrator, reading the text written by her husband.

When the symphony was subsequently recorded by the New York Philharmonic, when Bernstein was the orchestra's music director, she again was the narator. On that recording, which is on Columbia KS 6605, she gives clear evidenceof her gifts as a dramatic actress of power and sensitivity.