Andrew Young demonstrated his style of diplomacy today by meeting with two militant black African leaders who in the past have been unsparing in their criticism of U.S. policy on Africa.

First lunching with Sam Nujoma, leader of the Southwest African People's Organization, then calling on Marxist Nozambique's President Samora Machel, the U.S. ambassador to the United Nations seemed at ease although he obviously did not see fully eye-to-eye with either.

Young's two meetings nealty illustrated his success - so far at least - in dealing with black African radicals who until recently viewed the United States and its African policy with undisguised hositility and skepticism.

In a phrase underlining his change of heart about the Carter administration, Machel said today, "I am not saying the United States has a solution, but it has a great contribution to make. This is why I say welcome."

Just a few months ago, Machel was among the forefront of radical African leaders who said the United States had no role in helping solve southern African problems.

Nujoma had sharply denounced the United States only last week in Luanda, Angolo. He met here Sunday with representatives of five Western powers, including the United States, and then junked a tough draft speech denouncing the Western intiative in favor of a milder version.

Nujoma said his lunch meeting with Young was "a very useful exploratory contact" and Young said he and Nujoma had "begun to solve some problems of communication."

Here for a United Nations-sponsored conference on Rhodesia and Nambia (Southwest Africa), Young told Machel, "This administration has made a choice." He addes, "It is very clear that majority rule and freedom for all the people is something President Carter is committed to."

Young said, "I think we have come a very long way" since the Carter election victory in November. "I think the world does not yet appreciate how much different the United States can be not only in southern Africa but also in the Middle East, Vietnam . . ."

"President Carter has decided the United States should be with the people of the world," Young said to a finger-waving, friendly harangue from Machel, who said southern Africa was a problem "for our planet." To solve that problem, he said, "Is not an act of charity, but the duty if the whole of humanity."

Instead of listening to the public sessions with the anti-Western rhetoric that can be heard here as in most Third World conferences, Young seems to be concentrating on private meetings with leaders whose views went largely ignored under the previous administration.

His policy of aggressively seeking out and talking to African critics of the United States - instead of responding with hurt and anger - is also credited with persuading Britain, Frances, West Germany and Canada to attend this conference, which otherwise would have had only the traditional liberal Scandinavians as representation from the West.

Some reports also credited pressure from the United Evangelical Lutheran Church, a major force in Namibia, with influencing Nujoma to drop his hostility to the United States.

In today's speech, there was none of his customary language condemning the Western representatives' presence in Namibia as an "unfriendly act and a naked violation" of United Nations resolutions.

Nujoma said today that the Western representatives' contacts with the South African government were "very useful" but he still objected to their presence in Namibia.

"If the United States can exert pressure on South Africa we would be most grateful," Nujoma told Young.

Young's meeting with Nujoma reflected increasing Western belief that Namibia, long considered the easiest southern African problem, must be settled soon. Diplomats argue that South Africa is now ready to move toward granting independence to Namibia - albeit slowly and on the best terms it can extract - but even that could be reversed if the Rhodesian problem blows up and frightens Pretoria.