LOS ANGELES (AP) _ The Pentagon will investigate overcharging on some contracts by a TRW division in San Diego, the Los Angeles Times reported today.

Part of the investigation will examine TRW's own investigation, which led the company to return $2.5 million to the Defense Department.

''The inspector general has to verify it,'' a military aide in Defense Department Inspector General Derek J. Vander Shaaf's office told the newspaper.

''It could be an audit,'' the newspaper quoted the unidentified aide as saying. ''If it turns criminal, it could go to the DCIS (Defense Criminal Investigative Service).''

TRW reported Monday that it discovered ''instances of improper charging on certain government contracts'' in its San Diego military electronics and avionics division. The company said it disciplined 12 employees, including two managers who were fired.

The company did not identify them, but Hugo Poza, head of the San Diego department, told the Times he and group Vice President Robert L. North were fired.

The Times quoted North as saying the division where the overcharges occurred was one that he oversaw. ''In my opinion, the company is blaming employees for what was a deficiency in procedures and training prior to 1986,'' he said.

The overcharging, first reported in an anonymous letter to the Air Force in September 1984, occurred primarily during 1983 and 1984.