China has requested aid from the United Nations for the first time as the Peking government widens its efforts to find the capital for its enormous modernization effort.

For years, China emphatically rejected all foreign aid, insisting on paying its own way to development.

According to U.N. officials, however, China now is seeking not only aid from the U.N. Development Program but also has approached the World Health Organization and UNESCO.

While China has not specified the total of aid sought from the Development Program (UNDP), one official estimated that Peking would ask for about $100 million.

China has previously shown no interest in grants from the Development Program, although it has been a member of the U.N. agency since 1972. As a member, China has contributed a total of $7.5 million to the program and has allowed it to conduct training programs in China for technicians of other developing countries.

Since the early 1970s, China has accepted credit only indirectly, through deferred payments for purchases rather than direct borrowing from banks or foreign governments. One of the reasons for Japan's success in capturing a large share of China's trade has been the Japanese flexibility in arranging their transactions to suit the Chinese terms.

In the last six weeks, however, a number of foreign bankers have returned from China with word that Peking is going to raise capital directly on the international market.

Louis E. Saubolle, a vice president of the Bank of America, said China was negotiating a $1 billion loan with a group of Japanese banks and that a second loan would be arranged on the London Eurodollar market.

China's approach to UNDP - seeking grants far larger than the contributions it has made to the program - came last month, according to U.N. officials.

As a result of the approaches to the other U.N. agencies at the same time, representatives of WHO and UNESCO have already gone to China for preliminary talks.

UNDP, which does not make loans but gives money for development projects around the world, has a governing council that must approve any nation's application and set a dollar ceiling.

The 48-nation governing council is not scheduled to meet until June but UNDP officials will work with the Chinese until then to develop specific proposals for projects.

China's leaders have said they must modernize in four broad fields - agriculture, industry, defense and science and technology - after years of isolation and internal economic disruptions caused by political doctrine.

Any grant that UNDP provides, officials said, will be in industry and agriculture, since the U.N. agency never deals in defense matters nor with high-level technology that could be associated with weapons development.

Since 1972, China has hosted UNDP training programs in fish breeding, recycling of organic fertilizers for agriculture, soil management, weather and flood forecasting and acupuncture, according to UNDP officials.