Saudi Arabia has become the second largest creditor of the International Monetary Fund and therefore is likely to acquire its own seat on the IMF Executive Board late next year.

This would be a significant first step in bringing new powers - especially those in the Arab World - into the inner circles of decision-making in international economic affairs.

A high U.S. Treasury official told The Washington Post that the U.S. welcomes the Saudis to a new and more significant status in the IMF. "They play such a major role in the recycling of their (monetary) surplus, it makes sense for them to be on the board," he said.

But to add Saudi Arabia to the board, the IMF either must edge one current director off the board or expand it by one seat to 21 - a step the United States previously has opposed.

At present, there are 20 executives directors of the IMF, 5 of whom are appointed by member countries who make the largest quota contributions to the fund. In order of quota size, these countries are the United State, Great Britain, West Germany, France and Japan.

The other 15 are elected by groups of the other 126 countries in the international lending organization. The groups generally link countries on a geographical basis. In some, largest country in the group regularly names the director. In other, the honor rotates.

Saudi Arabia is currently 1 of 14 Arab or Moslem states, whose executive director at the moment is Muhammed Al-Atrash of the Syrian Arab republic.

Under IMF rules, if the two largest creditors of the IMF are not included among the appointed five directors, a creditor left out is entitled to his own seat. The United States, of course, is the largest bankroller for the IMF and, until recently, West Germany was the second largest.

But with the introduction of the so-called "oil facility" - a special pot of money loaned to the IMF beginning in 1974 - Saudi Arabia replaced West Germany as the second largest creditor. And last month, the Saudis further entrenched their position as a lender second only to the U.S. by committing themselves to a loan of $2.4 billion, out of a total of $10 billion, to a brand new fund to relieve balance-of-payments deficits.

This latter fund is called the Witteveen Facility, after H.J. Witteveen, the IMF managing director, who put the aid package together. All told, oil-producing countries are putting up 45 per cent of the new line of credit, while the industrial nations are advancing the remainder.

According to well-placed IMF officials who have discussed the matter with representatives of the Saudis, their government will be realy next year to claim its own seat when executive directors are selected in a routine biennial vote.

If things develop that way, Saudi Arabia in effect would become one of a new Big Six at the IMF, potentially outranking some of the others in real clout. In terms of monetary reserves, for example, the Saudis now hold $24.7 billion, second only to West Germany's $29.4 billion, and substantially larger than the U.S., Japanese, British, and French reserves.

IMF officials think that in its new role, the Saudis can be counted on to assume a greater responsibility for preserving the stability of the international monetary system.

Over a period of time, this could lead to a general realignment of the international economic power structure. A Trilateral Commission report last year urged that Iran, Brazil, and Mexico, as well as Saudi Arabia, be brought into the inner circle.

That report, written by economist C. Fred Bergsten, now Assistant Secretary of the Treasury for International Affairs, in effect said that the nations now dominating the power structure must face the facts of life about the post-1973 shift in the world's wealth and economic developments.

Bringing other countries into the top of the economic decision-making process "would avoid the risks that outsiders can disrupt the system," the Trilateral report said.

It suggested including Saudi Arabia among the rich nations' Group of Ten, long viewed as a steering committee for the Western - dominated international financial institutions, and inviting Iran, Brazil, and Mexico to join the 24-nation Organization for Economic Cooperation and Development in Paris.

A new Saudi IMF seat, however, will create some potentially touchy administrative problems, because it might force a regrouping of the 15 "clusters" of smaller countries into 14, edging one executive director off the board.

An alternative solution would be to expand the executive board to 21 - a step the U.S. always has opposed. But U.S. opposition to enlarging the the board has been based on objections to what it considered an "easy-way-out" approach to solving disputes over quotas and power relationships within the fund. The upcoming situation, where one of the two largest creditors does not have his own seat, is unique. So far, the U.S. reportedly has made no decision on whether to opt for a 20 or 21-man board.

If the board is held to 20, the presumption is that the executive director representing countries with the lowest aggregate of votes (which are related to quotas) would have to move off.

IMF sources point out that for that reason, when the question of a new seventh quota increase comes up during the next year, there will be considerable jockeying among the smaller countries to preserve their relative voting power, and to increase it if possible.

The group at the bottom of the ladder at the moment (not giving effect to the sixth quota increase) consists of a number of West African countries, whose director is Samuel Nana-Sinkam of Cameroon. Only fractionally larger in terms of votes is a group of Latin American countries, whose executive director is Dante Simone of Argentina.