One-fourth of the federal judges appointed by President Reagan in his first term were millionaires, according to a recent study by a University of Massachusetts political science professor.

In addition, 98 percent of Reagan's appointees were white, 98 percent were Republicans and 92 percent were male, according to the study by Prof. Sheldon Goldman.

But Reagan appointees had considerably more judicial experience than those of the four administrations that immediately preceded his, the study found. There also were fewer former prosecutors and political appointees among his appeals court choices, it said.

The study, to be published in the April issue of the monthly magazine Judicature, comes at a time when the administration is preparing to fill about 100 vacant judgeships. Together with 165 appointments in his first term, Reagan soon will have named about 270 of the nation's 744 federal trial and appellate judges -- or more than one-third.

By 1988, with additional appointments, Reagan will likely have picked more than half of the sitting federal judges -- including several Supreme Court justices -- and the new statistics give some indication of the kind of judiciary he will leave behind.

Goldman found that about 5 percent of the 187 judges appointed by Jimmy Carter in his last two years as president were millionaires, 20 percent were minority group members and more than 15 percent were women. About 90 percent were Democrats.

In an interview yesterday, Goldman estimated that 10 to 12 percent of the nation's sitting judges are women or members of minority groups. By the end of Reagan's second term, those percentages will be reduced substantially, he said.

The study shows an administration with an "absolutely extraordinary commitment and diligence" to ensuring ideologically conservative judges, Goldman said.

He called the Reagan administration the "most determined since the first Franklin D. Roosevelt administration" to mold a judiciary to its liking.

The Carter administration, on the other hand, was most committed to affirmative action, Goldman said, and ideology was not its sole, or even principal, concern. As a result, several Carter appointees to appeals courts who were minority group members or women were also conservative jurists, Goldman said.

Reagan has appointed two blacks and eight Hispanics to federal judgeships. Carter in four years appointed 37 blacks and 16 Hispanics in filling 214 vacancies.

The administration's "tenacity" shows up most clearly at the appeals court level, Goldman said, where the joint White House-Justice Department screening committee exercises its greatest control.

Senators often exert a considerable influence on selection of U.S. District Court judges in their states. But a president has much broader discretion in selecting judges for the 12 regional federal courts of appeal, which operate just one rung below the Supreme Court.

Of Reagan's 31 appointments to federal appeals courts, all were Republicans; 82 percent of Carter's were Democrats.

Reagan's appeals court selections show an administration committed to making "no mistakes" and to selecting judges with proven track records, Goldman said. That is why 70.9 percent of its selections at that level have had prior judicial experience, he said, compared with 53.6 percent of Carter's appointees.

Judicial experience, Goldman found, was far more important than the more traditional criteria of political activism or prosecutorial experience.

"You might expect this administration to favor former prosecutors," Goldman said, but Reagan, in his first four years, appointed fewer than his four predecessors: 19 percent. Nearly one-third of Carter's appeals court appointees were former prosecutors. Almost one-half of Richard M. Nixon's and Lyndon B. Johnson's appeals court judges were former prosecutors.

The percentage of prosecutors that Reagan appointed to district court judgeships was comparable to those of his predecessors.

If judicial experience is not available as a guide, the administration tends to look to law schools. Goldman found that 19.3 percent of Reagan appeals court appointees were law professors, compared with 14.3 percent for Carter appointees and fewer than 3 percent for both Nixon and Johnson.

Reagan is also less concerned with using judicial appointments to pay off political debts, Goldman found. About 58 percent of Reagan's appeals judges had been active politically, compared with 73 percent of Carter's appointees.

Reagan's appointees for the district and appeals courts are 50 years old on average -- almost identical to Carter's appointees. That finding is somewhat surprising given the administration's stated intention to appoint young judges who could influence the judiciary for generations.