The first memorial service for a victim of last week's Air Florida jetliner crash was held yesterday at an Annandale church where about 250 people gathered to hear eulogies for Donald P. Gilmore, a Postal Service Commission lawyer.

Gilmore, 31, was one of the younger passengers on the plane that plunged into the icy Potomac River last Wednesday, killing 78 people. Gilmore, who headed up the agency's payroll division, was traveling to Florida on government business, said his boss, James Finch, a senior assistant at the U.S. Postal Service Commision.

"Don was not the type of person who would brag about his duties," said Finch. "He was a smallish, quiet, very young man. When I first met him," quipped Finch, "I thought he was a summer intern." But, Finch added, Gilmore "was a person of great, great intellectual capacity . . . and dedicated to his work."

Finch, along with Gilmore's wife, Jan, and William Gardner, a minister who flew in from Gilmore's home town of Savannah, Ga., performed the hour-long ceremony at the Ravensworth Baptist Church.

Gilmore had served in the Air Force before joining the postal service in 1978. He will be buried in Savannah.

Little mention was made of the air tragedy during the service. Instead, the speakers focused on the young attorney's dedication and personal involvement with his work and friends.

"I sort of thought of him as a little scientist," said Jan Gilmore, who was married to her husband for 12 years.

"He would often take a problem--even my own minor problems--from a thesis to a solution, a positive solution," she said.

Jan Gilmore--a petite woman with short brown hair--recalled that she and her husband often were mistaken for brother and sister because they looked alike. "Many people confused us and called us 'Dan' and 'John,'" she related. The audience--friends, relatives, uniformed servicemen from Gilmore's Air Force reserve unit and pin-striped-suited coworkers--chuckled knowingly at the comment.

She grew more solemn during the end of her 15-minute eulogy, and tried to explain the meaning of the crash that killed her husband by citing a song written in 1977 by popular singer Billy Joel called, "Only the Good They Die Young."

"There was a record . . . called 'The Good They Die Young,' " she said, wiping tears from her face several times with the sleeve of her sweater. "Don was only 31," she sobbed. "I guess God seems to give the rest of us another chance."