Arline Van Blarcom Pratt, 78, an expert on State Department documents and a research assistant at the George C. Marshall Research Library for 20 years, died Feb. 26 at Alexandria Hospital following a stroke.

Mrs. Pratt was born in Natural Bridge, N.Y. She earned a bachelor's degree from Tufts College and a master's degree in history from Columbia University. She lived in New York City before coming to Washington about 1942.

She worked for a year for the legislative reference service of the Library of Congress and then for the division of historical policy research at the State Department.

In 1947, she began working for a series of well-known historians who were writing about the events of World War II and the years leading up to it. They included William Langer and Herbert Feis, winner of a Pulitzer Prize.

From 1954 to 1956, she worked with Dr. Philip Crowl to select papers of Secretary of State John Foster Dulles for microfilming for a profect at Princeton University.

In 1957, she joined Forrest C. Pogue, the authorized biographer of Gen. Marshall, at the Marshall Research Library. Dr. Pogue said that one of Mrs. Pratt's chief values as a research assistant was her intimate knowledge of State Department files covering a considerable period of the 20the century. She retired in 1977.

Her marriage to Malcolm F. Pratt ended in divorce.

Survivors include a brother, Vernon B.Van Blarcom, of Warren, N.J.

The family suggests that expressions of sympathy be in the form of contributions to the Alexandria Library, 717 Queen St., Alexandria, Attention: Mrs. Plitt.