Montgomery County Police Chief Robert J. DiGrazia yesterday dismissed Officer Joseph Baltimore from the force after a police trial board found that Baltimore had sexual relations with a 15-year-old girl while he was on duty.

DiGrazia, acting on a recommendation from the six-member trial board, agreed "with reservation" to award the officer more than $7,000 in back pay.

Baltimore, 32, a six year police veteran, has been suspended from the force without pay since being charged with raping the 15-year-old in the basement of a Bethesda elementary school in February.

Baltimore, who was assigned to the Bethesda station, was acquitted of rape during a six day court trial in July.

He was subsequently tried by the police board on administrative charges.

The board also cited Baltimore for "using his police authority" to solicit dates from two other 15-year-old girls in return for not arresting them on traffic violations.

Baltimore was guilty of conduct unbecoming a police officer and neglecting his police duties, the board said.

Courtland Townsend Jrs., Baltimore's attorney, said he would appeal DiGrazia's ruling to the Circuit Court.

Baltimore said yesterday that he thought the police hearing was "unfair" because board members examined only part of the testimony which was presented at the criminal trial. He maintained that the 15-year-old girl who charged him with rape had lied.

The girl testified that Baltimore raped her early Feb. 13 as she was showing him an area in the sub-basement of Brookmont Elementary School known as the "party room" - where youndsters reportedly went to drink and smoke marijuana.

Baltimore said he has not had a steady job since he was suspended from the police department, but has an income from a horse farm he rents in Upper Marlboro.

"I handled my job in the way I thought it should be handled. I was compassionate and trusting," Baltimore said.

Additionally, the police board said Baltimore violated the department's standing general orders by failing to notify the parents of the teen-aged girls he took into his police cruiser and by leaving his cruiser unattended without informing dispatches while in the subbasement of Brookmont Elementary.

A charge that Baltimore violated uniform regulations by not wearing his police hat at the elementary school was dismissed.

Daniel J. Cassidy, assistant county attorney, who represented the police at Baltimore's hearing, could not be reached for comment.