More than a half-dozen developing nations, including Brazil, the largest Third World debtor, may win substantial relief on their commercial bank debt within a year, a senior treasury department official suggested Wednesday.

Other candidates, said David Mulford, Treasury undersecretary for international affairs, are Ecuador, Uruguay, Bolivia, Nigeria, Morocco, the Ivory Coast and Poland.The relief would be negotiated within the framework of the so-called Brady Plan, named after Nicholas Brady, the U.S. Treasury secretary, who last year called on the International Monetary Fund, the World Bank and other agencies to help defuse the Third World debt problem by supporting debt relief packages.

Both Mr. Mulford and U.S. commercial bankers told a Senate Banking subcommittee Wednesday that the Brady plan is working.

They cited a recently concluded debt relief pact with Mexico and others

tentatively reached with the Philippines, Costa Rica and Venezuela.

Mr. Mulford and William Rhodes, Citibank's senior executive-international, lauded the outlines of the new Brazilian government's economic program.

Mr. Mulford termed it "bold and very strong." Mr. Rhodes, calling it ''bold and innovative," said that he hoped to meet soon with Brazilian government representatives to discuss a possible financial package.

If things go smoothly, said Mr. Mulford, Brazil and its creditor bankers might reach an agreement by the end of this year. Both Brazil, whose debt interest arrearages approximate $5 billion, and the bankers have a "strong incentive to move quickly," he said.

"In our view," he said, "Brazil will need a comprehensive, medium-term financing package, including a wide-ranging menu of options for debt and debt- service reduction. The first priority, however, is successful implementation of Brazil's economic program."

Mr. Mulford appeared less sanguine about Argentina, already $6 billion in arrears to commercial banks, and so far unable to mount a sustained economic program. A debt relief package for Argentina, he said, remains "some ways off."

On the other hand, he suggested, the chances are good that Uruguay, Ecuador and Morocco will have negotiated within six months debt relief packages with their commercial bank creditors.

A final agreement on the just-negotiated Venezuelan relief package, which is considered the most comprehensive Brady plan accord so far, will take several months, he indicated. Signing it by mid-summer would be ''impressive," he said.

In sum, he said, Third World debt is no longer a basic problem for the international financial system. He appeared especially optimistic about Mexico, which, he said, may by 1994 reach a 5 percent to 6 percent annual economic growth rate, about double last year's growth.

Mr. Rhodes, however, said he still harbors "major concerns" over Third World debt strategy, which, he said, has "positioned the debt crisis at a turning point."

The strategy, he said, needs greater financial backup from the industrial countries and more "flexibility" by the International Monetary Fund and World Bank in providing the backup. He also decried recent IMF-World bank policies of lending to countries in arrears to banks.