WASHINGTON (AP) _ A new process for evaluating ambassadorial nominees will improve the quality of U.S. diplomats and boost morale in the Foreign Service, two congressmen say.

In a joint statement Tuesday, Sens. Charles McC. Mathias Jr., R-Md., and Claiborne Pell, D-R.I., said four of their colleagues on the 17-member Senate Foreign Relations Committee, which approves ambassadorial appointments, also have agreed to participate in the new process.

The four others are Sens. Alan Cranston, D-Calif., Christopher J. Dodd, D- Conn., Daniel J. Evans, R-Wash., and Nancy L. Kassebaum, R-Kan., the statement said.

Such a review would be similar to the procedure used by the American Bar Association to screen nominees for the federal judiciary. The committee's screening panel will be the American Academy of Diplomacy, established last year by 68 former U.S. diplomats.

The screening process will be supported by a $100,000 grant from the Ford Foundation, said academy President David D. Newsom, a former under secretary of state for political affairs.

''Over time, we hope this procedure will raise the level of competence of ambassadors,'' said Mathias, who has complained about the qualifications of some political appointees. ''This should bring a tangible boost in the morale of the career Foreign Service.''