President-elect Ronald Reagan informed South Korea after his election that he, like President Carter, believes that the execution of that country's best known opposition politician would have serious consequences for U.S. relations, according to diplomatic sources.

This message regarding the fate of opposition leader Kim Dae Jung was passed to Korean President Chon Doo Hwan by Reagan aides in Washington, the sources said.

Carter administration officials had appealed to the Reagan camp to oppose the Kim execution, a very sensitive political matter in both capitals. The quick post-election response from the Reagan camp was greeted with relief in administration circles, especially because of the possibility that silence from Reagan would be interpreted in Seoul as acquiescence in the death of the opposition leader.

Assistant Secretary of State Richard Holbrooke, who has been spearheading the Carter administration's efforts, said yesterday that the Reagan camp's action regarding Kim was "thoughtful, helpful and well done."

The case of Kim, who was sentenced to death by a military court on Sept. 17 after a month-long trial on sedition charges, is now before the Korean Supreme Court. But is widely assumed that Chon, a former general who took office as president in August, will make the decision on Kim's fate.

While saying little or nothing in public about the case, the Carter administration has sent a stream of messages to the Korean leader expressing grave concern about Kim, both on humanitarian grounds and on the more pragmatic grounds of the damage his death would to do to Korea's standing in Congress, in U.S. public opinion and in the eyes of Japan and other allies.

Among other ways, the administration view has been expressed to a series of recent Washington visitors, including Kim Kyong Won, secretary general of the president office, and Gen. Lew Byung Hyun, chairman of the Joint Chiefs of Staff, who were here this week.

Reagan, while expressing general support for human rights, has been critical of the Carter administration's application of human rights concerns in friendly countries. This stand was widely interpreted at home and abroad as a carte blanche for repressive actions on the part of authoritarian regimes overseas.

The message to Korea on the Kim case, in this light, was a surprising indication that Reagan's words of support for human rights might be more than lip service. However, it was unclear whether this first practical application of Reagan's human rights policies would set the pattern for words and deeds to come.

For one thing, the message to Seoul was arranged by the Reagan transition staff at a time when the permanent foreign policy leaders of the new administration were not yet chosen. For another thing, the Kim case, involving a leading political figure known throughout the world, is unusually dramatic.