Merrill Donnaldson Knight Jr., 78, a civil engineer with the Federal Highway Administration's Bureau of Public Roads for more than 20 years, died of cancer Sunday at Circle Terrace Hospital in Alexandria.

Mr. Knight was born in Rockville, and earned a bachelor's degree at Washington College in Chestertown, Md.

From 1923 to 1935, he served as director of public works for Lynchburg, Va.

During World War II, Mr. Knight worked for the War Production Board here, then joined the Bureau of Public Roads in 1945. He retired in 1968.

He was a member of the American Society of Civil Engineers and St. Patrick's Episcopal Church in Falls Church.

He is survived by his wife, Alice H., of the home in Alexandria; a son, Merrill D. Ill, of Damascus, Md.; a daughter, Alice W. Glover, of Fredericksburg, Va.; two brothers T. H. Owen, of Silver Spring, and Albin S., of Asheville, N.C.; four sisters, Julia S., of Alexandria, Lucy Schwab, of Sedona, Ariz., Ann Bradofrd, of Amityville, N.Y., and Elizabeth Kasanin, of San Francisco, and six grandchildren.

The family suggests that expressions of sympathy be in the form of contributions in Mr. Knight's name to St. Patricks' Episcopal Church in Falls Church.