Last year John Harbison entered the Kennedy Center/Friedheim Awards Competition for American composers and did not make it to the semifinals. This year he did not enter the competition and won first prize.

"My publishers, Associated, sent it in without my knowing it," he said last night after the strain of waiting for the judges' decision was over and his piano concerto had won him the first prize of $5,000. "These competitions can be rough. It's especially hard for performers like the violinists last night."

Harbison is a tall, dark, lean 41 -- when asked his age, he said, "You had better ask my wife to be sure I get it right." He has taught at MIT in Cambridge since 1969 but will take a year off in 1981 to be resident composer at the American Academy in Rome. "The resident is a kind of overseer for the composers who are working there," was his explanation.

Harbison is no stranger to large awards. He has had commissions from the Naumburg, Koussevitsky and From foundations and the New York State Bar Association.

"I have been working to write music that is accessible," he said, speaking of the openly romantic cast to his prize-winning concerto. Having said in his notes that "the piano is no longer the ruler of the household and the concert stage," Harbison expanded on that idea. "The piano was designed for the things it was to do -- the 19th-century piano for sonority, a kind of power, an atmosphere, almost orchestral. The 20th-century piano changed to an abstract affair. I am trying to keep in mind some of the reasons that the piano took the shape it did."

Though he has been a jazz pianist, Harbison, says he is not a pianist these days. He is, however, the music director of the Cantata Singers and Ensemble in Boston, returning to that extra-curricular post after being away from it for several years. "I do it now about one-third of the time. I missed it."

His opera, "Winter's Tale" was given its premiere last year by the San Francisco Opera, and a chamber opera, "Full Moon in March" had its first performances by the Musica Viva of Boston this year. Harbison is now at work on an orchestral composition commissioned by the Boston Symphony in connection with the orchestra's 100th anniversary. It will be presented in the '81-82 season.