Katherine G. Chapin Biddle, 87, a widely published poet and the widow of former Attorney General Francis Biddle, died yesterday in Devon, Pa., after a long illness.

Mrs. Biddle, like her husband a thinker and a writer, had lived in Washington for 40 years, the last 30 in the couple's home at 1669 31st St., Georgetown. She moved to Pennsylvania in 1973 following a stroke. Her husband died in 1968, at age 82.

During World War II, while her husband served in the Roosevelt Cabinet, Mrs. Biddle received widespread acclaim for a number of her works, including "Plain-Chant For America," a long, moving poem that she called her reaffirmation of democracy. It was set to music by the noted composer William Grant Still and performed by the Philadlephia Orchestra and the New York Philharmonic and London Philharmonic orchestras.

In a 1943 interview, Mrs. Biddle said she had been inspired to write the poem five years earlier after an encounter with Fascist sympathizers. "It seemed to me high time somebody came forth with a reaffirmation of democracy, the philosophy of equal chance for every race, color and creed," she said.

Born in Connecticut, Mrs. Biddle was reared in New York City and studied privately under a number of prominent figures in music, theater and philosophy.

She was married in 1918 to a member of the rich and distinguished Biddle family, a man who was an honors graduate of Harvard and who at the time was a rising young Philadelphia attorney. The couple moved here in 1934 when Biddle, a fervent and effective Democrat, was appointed head of the National Labor Relations Board.

Mrs. Biddle, writing under the name Katherine Garrison Chapin, was first published in 1930 with "Outside of the World." a volume of poetry. A second book, "Times Has No Shadow," was published six years later.

Many of her most moving poems during this period, including "Bright Mariner," "New Year's Eve," and "With Long Remembered Light," dealt with the sorrow and acceptance of the death of a beloved young son.

In 1938, she wrote "Lament for the Stolen," expressing her feelings about the Lindbergh kidnaping. That also was set to music.Three years later, Still composed music for another of her works, "And They Lynched Him On A Tree," a work that was performed by the New York Philharmonic.

Her 1948 play, "Sojourner Truth," dealt tenderly with a black heroine who was a reformer during slavery. Her last volume of poetry, "The Other Journey," published in 1959, presented a number of earlier works as well as new pieces.

In addition to her books, and plays, Mrs. Biddle wrote poems and literary criticism for several magazines, including Harper's, Scribner's, Saturday Review, the New Republic, Poetry and American Scholar.

She was a judge for the National Book Award for poetry, the Bollingen Prize and the Shelley Memorial Award and as a consultant to the Library of Congress, gave a number of talks there on poetry.

She is survived by a son, Edmund R., a professor of English, and his wife, of Bala-Cynwyd, Pa., and two grandchildren.