Lawyer William T. Newman Jr. will become chairman of the Arlington County Board Jan. 1, his colleagues announced yesterday, making him the first black person in this century to hold the post.

With county officials predicting a $30 million shortfall in projected revenue next year, board members have said some services may be cut.

"The County Board is going to have to do a lot of reflection," Newman said yesterday at a news conference.

Four black members were on the board from 1871 to 1888 and for brief periods held a majority when the county was called Alexandria County, according to Arlington Historical Society records. Records are unclear on whether any black members served as board chairman.

Asked whether he attached significance to his historic status as a black politician, Newman said, "Yes and no. It's there and certainly a part of the fabric of the history of our community. But it's not the same as someplace else," where race relations are more strained. "I've always felt a part of the community."

Newman, 40, was elected in 1987 and is serving his first term on the board. A former actor who once had a part in a soap opera, Newman has gained a reputation on the board as affable and quietly dedicated. He has devoted himself to issues such as public safety and drug abuse control, and with his prodding, the county established a human rights commission last year that reviews complaints of discrimination.

"He's very deliberate," said civic activist Louise Chesnut, a longtime board observer. "In the beginning, he quietly watched and learned. Now he's taking his place on the board."

The board chairman post is a largely ceremonial but high profile one. The chairman presides at the board's twice monthly Saturday meeting, and often is called upon to speak for the board. The chairman makes an annual salary of $15,601, $1,559 more than board members.

In recent years, it has been common for the five-member board of four Democrats and an independent endorsed by the Democrats to confer the chairmanship on a colleague up for reelection. The terms of both Newman and current board Chairman Albert C. Eisenberg end next year. They declined to say yesterday whether they will run again.

In addition to deciding whether to cut services dear to many Arlingtonians, the board will be choosing a site for a controversial drug detoxification center and shelter for the homeless. A citizen panel has recommended three possibilities, one of them near Newman's neighborhood of Arlington View in South Arlington.

Newman's selection as chairman, and the selection of board member Ellen M. Bozman to succeed Newman as vice chairman, is to be formally ratified at the board's annual organizational meeting New Year's Day.