Conservative economist William A. Niskanen Jr., presently on the business school faculty of the University of California, is slated to become a member of the President's Council of Economic Advisers.

CEA Chairman Murray L. Weidenbaum would neither confirm nor deny Niskanen's prospective appointment. But it was learned that Niskanen, 48, will actually begin his service today as a CEA consultant. Including Weidenbaum, the CEA is a three-person advisory group. One post has yet to be filled in addition to that Niskanen would hold.

Government officials say Niskanen is a micro rather than a macro-economist, meaning that he is likely to specialize in specific problem areas, rather than deal with the overall economy. Like Weidenbaum, Niskanen has had a special interest in government regulation and deregulation. During the recent presidential campaign, Niskanen was a member of Weidenbaum's task force on government regulation problems.

He made a few headlines last year when the story surfaced that the Ford Motor Co. had fired him as its chief economist for advising the company that government-imposed quotas on Japanese cars would not solve Ford's problems.

According to a Wall Street Journal story, when Ford rejected Niskanen's counsel and decided to abandon its traditional free-trade stance by asking for help from Washington, an official told Niskanen:

"Bill, in general, people who do well in this company wait until they hear their superiors express their view and then contribute something in support of that view."

Niskanen said at that time that import competition represented only about one-eighth of the loss of Ford business over the prior two years, with the rest traceable to recession and demands for small cars that Ford wasn't producing.

Coincidentally, in the current discussions by President Reagan's task force debating the auto industry problem, the same one-eighth figure has cropped up as an estimate of the probable relationship of Japanese imports to the problems of the industry overall.

Niskanen's conservative, smaller-government orientation dovetails neatly with that of most of the Reagan economists. Some liberal Democratic economists, who would not be quoted directly, criticized him yesterday as "opinionated and assertive," and spoke bitingly of his "antigovernment religion."

Niskanen received his doctorate in economics at the University of Chicago in 1962, having previously received a master's degree at Chicago and a bachelor's degree at Harvard. Prior government service included the OMB (1970-72) as an assistant director and the staff of the Office of the secretary of Defense (1962-64).