The Justice Department will not prosecute three Fairfax County police officers accused of needlessly beating a 16-year-old they were arresting, a spokesman said yesterday. A probe of an internal police trial board that cleared one of the officers also has been dropped.

A Justice Department official said yesterday that there was insufficient evidence to prosecute the police for violating the youth's civil rights. "There's nothing to go on," the official said.

The official would not say if the decision to drop the investigation came from Justice Department officials or a federal grand jury in Alexandria that began investigating the incident in June.

Thomas H. Dameron, attorney for the youth, Howard L. Cadle, said yesterday that he was surprised at the Justice Department's decision. "I felt like they had the evidence to go ahead and prosecute," Dameron said.

The decision "certainly can't help" the $200,000 civil law suit Cadle's father filed against the policemen as a result of the alleged beating, Dameron said. That suit is scheduled for trial Sept. 21 in U.S. District Court in Alexandria.

Fairfax County Police Chief Richard A. King said he was glad the men and the trial board were cleared and that "the grand jury and the Justice Department came back with the same view" he had.

The incident, which a county attorney said marked the first time federal authorities had investigated the county police for their conduct, grew out of the Jan. 6 arrest of Cadle at the Franconia roller rink. Cadle was being sought at the time as a runaway, police said.

Witnesses to the arrest said they saw policemen hit Cadle repeatedly on the head and shoulders with night-sticks.

Cadle's father, Lacy, brough brutality charges against the three officers, identified as Keith M. Florence, Robert T. Hubbard and James F. Kelly Jr., but the charges against Hubbard and Kelly were dismissed by King.

The charge against Florence was heard by a police trial board March 29. The board, composed of police officers, recommended that Florence be cleared and King dismissed the charge the same day.

All three officers are now working on the force.

In a separate case, the police department agreed last February to pay $25,000 to a woman whose husband was shot to death by a county policeman following a high speed chase.

This week a Falls Church man who said police shot him in the leg and stomach during a traffic arrest agreed to drop his suit against the police department, King said.