The postman charged with aiding the assassin of prominent Iranian exile leader Ali Akbar Tabatabai in Bethesda last summer pleaded guilty in federal court here yesterday to a charge that he agreed to accept $500 for the use of his Postal Service jeep in the murder plot.

Tyrone Anthony Frazier, 31, also has agreed to cooperate with prosecutors in the investigation of the murder and will be allowed to enter the federal witness protection program, according to a plea agreement read in open court yesterday. This program is designed to safeguard the lives of government informants in sensitive cases.

In entering his plea before U.S District Judge John J. Sirica, Frazier became the first person charged in the slaying to plead guilty. Two other men have been charged in the murder.

Tabatabai, an outspoken critic of the Iranian regime of Ayatollah Ruhollah Khomeini, was shot on the doorstep of his home July 22 as he accepted an assassin who was dressed like a postal carrier. Authorities have said that the bogus mailman was Daoud Salahuddin, whose name was David Belfield before his conversion to Islam.

Salahuddin, a member of a small militant group of black American Muslims who have adopted the Khomeini brand of revolutionary Islam, has been charged with the murder of Tabatabai but never has been arrested. Investigators believe he is living in Iran.

In accepting Frazier's plea to the charge of accepting an illegal gratuity, prosecutors agreed to drop a more serious federal bribery charge. Frazier faces a maximum sentence of two years in prison and a $10,000 fine.

Frazier, who initially told investigators that his truck was taken against his will, later admitted that he had lied and said he agreed to give the truck to Salahuddin, a man he knew casually, prosecutor E. Lawrence Barcella Jr., said. After Frazier turned over the jeep to Salahuddin the day of the murder, a third person, identified as Horace Butler, drove Frazier to Baltimore and around Maryland for four hours before dropping him off, Barcella said.

Butler, a carpenter, is awaiting trial on charges that he conspired to assist in the murder.