A 19-year-old Catholic University student last night plunged to his death from the window of a fifth floor dormitory room during a small party celebrating the end of final examinations, District of Columbia police said.

Police said James Armstrong, a sophomore from Pearl River, N.Y., was attending a party with friends in a room at Flather Hall on Catholic University's Northeast Washington campus about 8:30 p.m. Police said the youth apparently leaned out of a window to knock on the window of the room next door and lost his balance.

Armstrong fell on top of a concrete canopy beneath the window and then onto a dirt and brick area below, police said. Police officials ruled the death an accident and said no foul play was suspected.

Police said the exact nature of Armstrong's injuries would not be known until the city medical examiner's report is completed.

In similar accidents on local college campuses, a 19-year-old Georgetown University student fell four stories to his death in September during a series of Labor Day weekend parties, and in January 1980, a 19-year-old Catholic University student survived a fall of 65 feet from a campus building after spending an evening at a beer-and-gin party.