The long-awaited trial of five national guardsmen in El Salvador charged with murdering four American women missionaries two years ago will be postponed for about two months, according to the Associated Press, while judges consider a request for dismissal of the proceedings.

The defense lawyer has argued that Salvadoran Judge Bernardo Rauda Murcia had set the trial date for the five soldiers before their defense lawyers were appointed.

The postponement will delay start of the trial well beyond the Jan. 23 date on which the Reagan administration must certify to Congress that El Salvador is improving its human rights record if it is to continue to receive aid from this country.

Congress has made that assurance a requirement before it approves $61.3 million in military aid and $164.9 million in economic aid from the United States to El Salvador next year.

An estimated 30,000 people have been murdered over the past three years in fighting between guerrillas and government forces there.

Earlier this month, relatives of the slain Roman Catholic missionaries denounced the trial as an attempted "cover-up," maintaining that the killing of the four women had been ordered at a higher level in the government.