President Carter today nominated Donald F. McHenry, a black career diplomat, as the new U.S. ambassador to the United Nations, praising him as a "professional" who can continue the Third World relations forged by his predecessor, Andrew Young.

Carter said that McHenry had been "highly recommended" by Young and Secretary of State Cyrus R. Vance, McHenry had been serving as Young's McHenry had been serving as Young's deputy representative to the U.N. Security Council, having the rank of ambassador.

"He's a highly qualified professional who is familiar with the major issues before the United Nations," Carter told reports. "I have complete confidence in him."

Many times during the past 2 1/2 years, Carter found himself wincing -- and then having to explain -- diplomatic gaffes by his friend Young, who submitted his resignation as U.N. ambassador earlier this month. Carter has privately made it clear that these comments annoyed him and detracted from Young's accomplishments in improving ties with the Third World, particularly in black Africa.

Young was forced to submit his resignation earlier this month after it was disclosed that he met with a representative of the Palestine Liberation Organization, in violation of U.S. policy, and then misled the State Department about it.

It was clear today that Carter was taking considerable comfort in having selected as Young's successor a man who is a skilled career diplomat and who has close ties of his own to the Third World.

"Ambassador Don McHenry fulfills all the requirements, I think, for a superb appointment," Carter said. "His whole life has been devoted to ambassadorial or diplomatic service . . . He is a man that I have known very closely since I have been president, because of his work in southern Africa in particular."

Carter said he believes that McHenry's nomination "would guarantee" that present U.S. relations with the Third World countries would continue.

Carter said that he had considered others for the U.N. post including U.S. Ambassador to China Leonard Woodcock. The president said he did not choose Woodcock because he believes it is important to continue present U.S. policy toward China. And he added that McHenry was "recommended to me as the first choice of a lot of people."

At a news conference in New York after his nomination was announced, McHenry refused to say whether he would meet with the PLO, as Young had.

But McHenry lauded Young's policies and, in response to a question, said he too might find it necessary to take actions that are against State Department policy.

"We all have to follow the dictates of our own conscience," McHenry said, adding that every ambassador "faces a point when he has to decide how a particular policy goal" is best carried out.

The ambassador, however, must realize that "he has to face the music if he does so . . . That will be my own philosophy," McHenry said.

McHenry indicated, without saying so directly, that he agreed with Young's assessment that it is foolish for the United States to refuse to talk to PLO representatives.

In response to a direct question, he said the "unique value" of the United Nations is as a forum for communications and that "communication is important in the resolution of problems in general, be they between nations or between father and daughter."

He said that the Camp David accords make it clear the United States will have to "find some . . . way of including the Palestinians in the search for peace in the Middle East," although he was careful not to mention the PLO.

McHenry, whose low-key approach is in marked contrast to Young's charisma told reporters he did not feel he had "shoes to fill" and stressed he will do things "in my own way."

Does that mean a non-charismatic approach?" he was asked. "In my own way," he repeated.

At a luncheon he hosted for the U.N. Security Council an hour before the news conference, Young praised McHenry and said, "We will still be around in the background on call whenever he needs help."

McHenry told reporters that he felt Young could be helpful and "I hope I have the good sense to listen to his advice."

McHenry rejected suggestions that he was nominated because of his experience in Africa or his role in negotiating with Soviet officials last week over the departure from the United States of Soviet ballerine Ludmilla Vlasova whose husband, Bolshoi Ballet star Alexander Godunov, had defected earlier.

Nor did he think his appointment was aimed at easing the tensions between the black and Jewish communities over Young's resignation.

McHenry, 42, a St. Louis native, studied international relations at Southern Illinois University and Georgetown University.

He joined the State Department in 1964 and worked on international policy questions until he resigned when Henry A. Kissinger became secretary in 1973.

McHenry was at the Brookings Institution and the Carnegie Endowment for International Peace until 1977, when he joined the Carter administration.

While he was Young's deputy, he served as chief U.S. negotiator on the U.N. group that put into motion the process for Southwest Africa (Nambia) to become independent after being controlled by South Africa for 59 years.

McHenry also has dealt with Southeast Asia and the Middle East as well as Africa, and this year was involved in outer space and disarmament questions.

In Moscow, the Soviet news agency Tass today blasted McHenry for his role in the Vlasova negotiations, saying he refused to answer questions from Soviets at the scene and "evaded explaining" on what grounds the United States was blocking the Aeroflot plane from taking off.

Later, Tass announced McHenry's nomination to succeed Young in a three-paragraph story that did not mention the airport incident.