Frank A. Weil, the outspoken assistant secretary of commerce, wrote Commerce Secretary Juanita Kreps today that he will resign his post by Oct. 1, Commerce Department sources said.

Weil, formerly a New York businessman, is being considered as a replacement for Robert Strauss as the president's Special Trade Representative.

Administration sources said that besides Weil, the White House is considering John L. Moore, Jr., head of the Export Import Bank and Lloyd Hackler, president of the American Retail Federation.

"I'm not saying that one of the three of them will be a winner," said one highly placed administration official, "but that's the best list in town."

Strauss has said that he will leave the post of Special Trade Representative as soon as Congress completes consideration of legislation necessary to implement the massive multilateral treaty to reduce barriers to trade that major nations agreed to early this year.

The treaty has intensified debate over how the nation carries out its international trade policy. Now at least five agencies - Treasury, State, Commerce, Labor and the Special Trade Representative's office - have their hands in trade policy.

The White House officially is backing a plan that would concentrate most trade duties in the Department of Commerce, although sources say that the proposal is losing steam as top administration officials worry that trade policy would be impeded, not helped by placing it in the already unwiedy and cumbersome Commerce Department.

Sources said that Weil, in a leter to Kreps today, said he did not plan to stay much beyond October 1, even if Commerce's role in trade policy were strengthened.

Weil, as assistant secretary for industry and trade, presumably would play a strong role in a beefed-up trade area in Commerce.

But sources in the department said that Weil has been unhappy for some time with Kreps and her top aides and that the unhappiness flows in the opposite direction as well.

Weil, in his two years as assistant secretary, has been a strong advocate of the need for U. U. industry and the federal government to increase the effort to sell U.S. goods abroad. The United States, he has complained, is the only major industrial nation without a ministry of foreign trade.

The Special Trade Representative's office, which has cabinet rank, has negotiating and some enforcement duties, but what little export promotion the U.S. does comes from the Commerce Department.

Although Weil is dissatisfied with Commerce, he apparently is willing to consider other administration posts and wants the STR job. White House sources said he is not considered a front runner to replace Strauss - who will concentrate on Middle East problems after Labor Day - but that Weil is not out of contention either.

Moore, a former Atlanta lawyer, has headed the Export-Import Bank since March 1977.The agency assists in the financing of U. S. exports.

Hackler, a former administrative assistant to Sen. Lloyd Bentsen (D-Tex.) and an assistant press secretary to President Johnson, is head of the umbrella trade group for major retailers. He also served on Strauss' advisory group for the multilateral trade talks and Strauss is said to number him among his chief links to private business.