President Reagan yesterday told the mother of murdered Navy diver Robert D. Stethem that Mohammed Ali Hamadei, accused of killing her son, will be brought to trial in West Germany and "get the justice he deserves."

Reagan sought to reassure Patricia Stethem, who said her family was "devastated" by the West German government's refusal to extradite Hamadei and criticized the president's efforts in the case as insufficient.

Stethem, of Waldorf, Md., was a passenger on a TWA airliner that was hijacked two years ago. The four Shiite Moslem hijackers, one of whom allegedly was Hamadei, beat Stethem to death and held 39 other Americans prisoner in Lebanon for 17 days.

Washington Post correspondent Robert J. McCartney reported from Bonn that senior chancellery official Wolfgang Schaeuble said that the West German government had decided against extradition to "reduce the risks" facing two West German businessmen held by kidnapers in Lebanon seeking Hamadei's release. Schaeuble said the government pledged to seek a life sentence for Hamadei, 22, and ruled out any swap of him for the two West Germans.

"I have the personal assurances of Chancellor {Helmut} Kohl that justice will be done," White House spokesman Marlin Fitzwater quoted Reagan as telling Patricia Stethem in a telephone call. "The chancellor has assured us that there will be no deals and they will not grant him clemency."

On June 10, Fitzwater said that the West German government had refused to extradite Hamadei despite a personal appeal from the president to Kohl during a dinner at the economic summit in Venice. But Reagan backed away from that statement at a news conference the following day, declaring he had not "attempted to put any pressure on Kohl" and expressing confidence in whatever decision the West German government made.

Stethem took issue with Reagan's approach in an appearance yesterday morning on NBC's "Today" show.

"Saying that he would be satisfied with whatever the German government did does not in my opinion seem like he made a personal appeal," she said. "It was more or less like he was saying, 'You take the responsibility.' "

This televised statement prompted concern among White House officials, who have been extremely sensitive about the political implications of the Hamadei case. Some of these officials acknowledge privately that U.S. leverage on its allies in cases where hostages are involved has been weakened by the sale of U.S. arms to Iran as part of an attempted swap for American hostages in Lebanon.

The Hamadei decision dominated discussion at the White House senior staff meeting, sources said, and officials quickly drafted a statement stressing West German determination to prosecute Hamadei. National security adviser Frank C. Carlucci and Fitzwater also urged the president to call Stethem and reassure her that there would be no trade of Hamadei for the West German hostages, these sources said.

In Bonn, Schaeuble made the same point, saying that "the German government is not open to any kind of blackmail, {and} we are not considering, have not considered and will not consider any exchange between Hamadei and the two Germans."

Attorney General Edwin Meese III, who had flown to West Germany in an unsuccessful last-ditch attempt to persuade officials to extradite Hamadei, issued a statement in Hamburg saying that Bonn's decision "demonstrates this government's full commitment to the rule of law and to international cooperation against terrorism."

But State Department spokeswoman Phyllis Oakley reflected some of the U.S. disappointment at the outcome in her briefing, saying "we preferred extradition . . . . " She added, however, that the decision "satisfies our concerns."

In another development, the White House announced that Reagan is to undergo a routine colon examination Friday as a follow-up to colon cancer surgery two years ago.Staff writers David B. Ottaway and Howard Kurtz contributed to this report.