Stephen H. Detwiler, the former Republican-backed chairman of the Arlington County Board who was upset in his bid for reelection last year, said yesterday he will not seek to return to the board this fall.

Detwiler, 39, a moderate who until recently was viewed as the Arlington GOP's most promising politician, said his appointment this week as chief operating officer of Continental Federal Savings and Loan will keep him from devoting much time to politics.

"I really felt that I had gotten into politics sooner than I thought I would," Detwiler said. "I consider myself young enough that there will be opportunities in the future where a career and public office can work together . . . I want to continue to have an active involvement in county politics, and we'll see what happens in the future."

Detwiler's announcement seems to remove an obstacle to reelection bids by the County Board's two conservative Republican-backed incumbents, Walter L. Frankland Jr. and Dorothy T. Grotos. Both have declined to say whether they intend to run again, prompting GOP speculation that they were waiting for news of Detwiler's plans.

Both said yesterday that Detwiler's announcement will not influence their decisions. "My decision would be an independent decision regardless of what Walter or Steve decide to do," Grotos said.

The Arlington Republican organization has suffered losses at the polls in recent years and has been criticized for taking what some party members see as an overly conservative stance in moderate Arlington.

With Detwiler's defeat last year, the Arlington GOP lost its four-year control of the County Board. The same election saw the defeat of the only candidate the county Republican Party was able to field for three House of Delegates races. In 1981, the party lost three Virginia House races.

Neither Republicans nor Democrats expressed much surprise at the announcement by Detwiler, who in recent months has sought to distance himself from political activities. Some Republicans also said they would not be surprised if either Frankland or Grotos decides soon not to seek reelection.

"If both had a choice, neither would run," said 10th District Republican Chairman Robert R. Bocek about the two eight-year incumbents. "I would not like it if both of them were to pull out. I think one of the reasons for the delay in their annnouncements is that both are not looking forward to running again."

Bocek said he is confident that Arlington Republicans will put up strong candidates if either incumbent decides not to run again, but declined to mention any prospects.

County Democrats are already queuing up for a shot at the board's two GOP-controlled seats. Last week, Albert C. Eisenberg, 36, a longtime Democratic activist, announced his candidacy. Richard Buffum, a retired CIA employe, has said he expects to announce his candidacy in a few weeks.

A story in today's Virginia Weekly says Detwiler had not yet ruled out another run for office. The Weekly was printed before Detwiler announced he would not be a candidate.