Virginia Gov. Charles S. Robb announced yesterday he will join the 300-member law firm of Hunton & Williams, one of the nation's largest, when his term expires Jan. 11.

The firm, with offices throughout the South, is among the 30 largest firms in the country and represents a wide range of national and regional clients, including Allied Corp., Virginia Power and other electric companies, as well as the Ethyl Corp. and Chesapeake Corp.

Robb, 46, will become a partner in the firm and will have offices in Washington, Northern Virginia and Norfolk, according to W. Taylor Reveley III, managing partner of the Richmond-based firm.

"We have a number of partners who have offices" in two cities, Reveley said. "Gov. Robb will be the first one to have offices in three."

Robb, whose salary was not announced, previously had said he would return to his home on Chain Bridge Road in McLean with his wife and children. There had been speculation that Robb might join the Alexandria law firm of William Thomas, a close political friend, but the governor had kept his plans secret.

Robb, who will be succeeded by Democrat Gerald L. Baliles, is thought by many to have national political ambitions. The governor, who has traveled extensively during his four years in office, is one of several southern and western state Democrats who are trying to moderate the image of the national Democratic Party.

His new job will give him access to Washington and an opportunity to travel extensively in the South, where many party leaders see him as a strong candidate for vice president, should he decide to remain in politics.

A graduate of the University of Virginia Law School, Robb will be the first former governor to join Hunton & Williams, a firm whose former partners include Supreme Court Justice Lewis Powell and Virginia Supreme Court Justice John Charles Thomas. Appointed by Robb, Thomas is the first black to sit on the state's highest court.

Former Virginia Power board Chairman T. Justin Moore also is a member of the firm. Moore has served on a variety of projects for Robb, including a task force on science and technology that resulted in the establishment of the Center for Innovative Technology in Northern Virginia.

Reveley said details of Robb's assignments in the firm are still being developed. "He'll be doing essentially what ever other partners do," he said.