President Reagan announced yesterday that he is nominating Stephen F. Williams, a University of Colorado law professor who is an expert in administrative and energy law, to a vacancy on the U.S. Court of Appeals here.

Williams, if confirmed by the Senate, would be Reagan's sixth appointee to the 12-member court -- an appointment that would assure a dominant position for the Reagan nominees on a court that for years was a bastion for liberal judicial activism. The court is often described as second in power and authority only to the Supreme Court, with a broad jurisdiction over civil rights, environmental and regulatory controversies.

The administration announcement followed the withdrawal of its previous nominee, former Office of Management and Budget general counsel Michael J. Horowitz. Horowitz, a prominent conservative, took himself out of the running two weeks ago, citing personal and professional concerns. But his withdrawal averted what many predicted would be a particularly bloody Senate confirmation battle.

Nan Aron of the Alliance for Justice's judicial selection project said Williams' nomination was another example of Reagan's "filling a slot on the Court of Appeals with another academic whose conservative philosophies are well-defined."

D.C. Bar President Frederick B. Abramson, who learned of the nomination only after it was announced by the White House, expressed concern that the administration had failed again to check with area bar leaders about a nomination for the local circuit. Bar leaders were angered when former senator James L. Buckley was nominated to the same court without consultation.

Williams, 49, a Harvard Law graduate and a professor in Colorado since 1969, is a firm advocate of energy deregulation and has criticized the Federal Energy Regulatory Commission for not allowing market competition to set prices for natural gas.

Williams' constitutional writing has taken a "restrictive view of the reach of due process protections," generally compatible with recent Supreme Court rulings, said Harvard law professor Richard Stewart. Stewart said Williams has "strong conservative views" but also is "very undogmatic, an open-minded, fair-minded person."