President Carter announced that he and South Korean President Park Chung Hee will propose that their two countries meet jointly with North Korea to open talks on the eventual reunification of the divided Korean peninsula.

The proposal represents a concession on the part of South Korea, which in the past has been cool to the idea of tripartite negotiations.

The new plan, which was suggested to Park by Carter earlier this year, will appear in a joint communique to be issued here by the two leaders today at the conclusion of Carter's state visit to South Korea.

Diplomatic sources, who revealed the plans Saturday night, said that neither country has any assurance of what the North Korean reaction to the proposal will be. One official, characterizing the prospects of a favorable response as "50-50," said, "The straws in the wind indicate that this may work."

North Korea is to be notified of the proposal this morning through third-party channels. In addition, sources said, both of North Korea's patrons, China and the Soviet Union, were being notified before today's announcement and were being asked to use their influence to bring about the talks.

"It's a serious proposal, one that we have thought a long time about," said one official. "We hope it leads to a lessening of tensions."

One source stressed that the joint proposal should not be regarded as a propaganda gesture by the United States and South Korea. He added, however, that it will obviously put the North Koreans under international public pressure to respond favorably.

Key members of the U.S. Congress were also notified of the move, the source said.

The communique will not specify a time or place for the proposed negotiations, and will describe the purpose of the talks in general terms. The sources estimated, however, that it would take about two months after a favorable North Korean response before negotiations could be under way. The site of the talks is likely to be a third country where both North Korea and South Korea have ambassadors, they said.

The U.S. and South Korean proposal will call for the talks to be held at the ambassadorial level, sources said.

Carter discussed the proposal Saturday in his brief discussion with Park on the first day of the state visit.

The proposal will be made in an effort to get substantive discussions started on reunification of the two Koreas, which have been divided under a military armistice since 1952. They do not recognize each other's legal existence.

North Korea always has insisted on talks directly between it and the United States, pointing out that South Korea was not a party to the armistice agreement.

South Korea, meanwhile, has in general preferred two-way talks between itself and the North and had not shifted from that position until Park agreed this weekend to Carter's proposal for tripartite discussions.

The South Korean government appears still to insist, however, that the main portion of the talks consist of two-way discussions between the North and South. Foreign Minister Park Tong Jin was quoted this week in Seoul as saying that if any new formulation were adopted "South and North Korea should be the main parties to the talks. This principle should be fully respected."

That led to speculation that the United States may have agreed to gradually withdraw from talks once they get under way, but diplomatic sources insisted that is not the American intention.

The South Korean government also sought to head off assumptions that it is making a major concession by agreeing to three-way talks. One government official said early this week: "We never said we would not sit down with a three-way meeting. What's important is that any talks should start from the basic theory that progress can only come through dialogue between two Korean parties."

The modest optimism that talks might actually get under way is based on what are regarded as encouraging signals coming recently from Pyongyang, including one which suggested that North Korea is no longer so firmly opposed to talking with the South.

North Korean Foreign Minister Ho Dam said recently that the North might agree to tripartite talks if the South changes its stand "radically" and really wants reunification. He suggested the South might "attend our talks with the U.S. when it reaches a definite state."

The diplomatic sources in Seoul said the communique today will not specify exactly what matters would be taken up if the talks take place and said it will be merely a "general formulation."

The aim, they said, is reunification of Korea into a single country but they explained that less difficult issues could be taken up along the way.

In their recent, inconclusive meetings, North Korea and South Korea differed radically over what they would talk about if they could sit down to negotiate. The South prefers to start with small and relatively uncontroversial issues such as the reunion of families and exchange of mail between the two countries.

The North, however, has insisted that the negotiations should immediately concern only the specific issue of reunification. CAPTION: Picture, The Carters, joined by President Park, wave during a motorcade through South Korean capital on Saturday.