In a surprise move, Arlington Chief of Police Roy C. McLaren, a nationally known expert in police administration who has been criticized as a stranger among his men, yesterday announced his retirement from the force effective Oct. 1.

McLaren said he had informed some senior officers about his decision and planned to announce it to uniformed officers at roll call today. But at least one captain said he was "shocked" at the decision when told by a reporter last night. Assistant County Manager Oliver S. Merriam also expressed surprise.

County Manager W. Vernon Ford was on vacation yesterday, and unavailable for comment.

McLaren said, however, that he informed Ford of his decision last week. It will be up to Ford to appoint a successor. Although McLaren was the first chief selected from outside the department, candidates from both inside and outside have been interviewed in past selection processes.

McLaren, 51, said he is leaving the Arlington job to participate in a one-year program of graduate study sponsored by the Police Executive Research Forum at Cranfield Institute of Technology in Bedford, England. He said he plans to study recruitment and selection procedures for officers, including testing methods.

Sgt. William C. Jeunette, who said earlier this year that "nobody knows the real Roy McLaren," said yesterday he was surprised at the announcement.

"I never heard a whisper," said Jeunette, who is president of the Arlington Police Beneficiary Association. "I'm sorry to hear it. We were just getting used to him.

"(McLaren's tenure) had some very valuable points, and we've had some low points, he added. "I don't think we've gone backwards - we haven't made any great strides either."

Criticisms of McLaren is the ranks have centered on his ostensible lack of rapport with his men. "He came in and remained in his ivory tower, so to speak," one officer said recently. "His main (problem) is his personality; he's not a warm person."

Other officers complained that they rarely saw or talked to the chief.

McLaren denied yesterday that the criticisms had prompted his decision, and said no one had pressured him to resign.

McLaren said he was "excited" about the prospect of going to England to study and said he had considered applying for the program last year. He applied about two months ago, but his acceptance was not final until Monday, he said.

McLaren, who was appointed Arlington chief in June 1973, is the senior tenured police chief among major Washington metropolitan jurisdictions.

In an interview last month, county manager Ford said McLaren's reputation is "impeccable."

Along with O. W. Wilson, McLaren wrote "Police Administration," a standard textbook often referred to as the "bible" of police manuals.

Before coming to Arlington, McLaren served 10 years with the International Association of Chiefs of Police as a police management consultant and director of field operations. At the LACP McLaren directed surveys of police departments across the country including New York, Detroit, Chicago, Atlanta and Pittsburgh.

McLaren still receives numerous requests for consulting work from other cities and often accepts the jobs and donates the money to the police department for equipment purchases. According to county figures, his work last year brought in $810 for the department.

Yet within the 377-member police department, officers said they hardly knew the man who was their chief. For many officers McLaren's show-down last month with Commonwealth's Attorney William S. Burroughs Jr. was the first sign that McLaren was the strong, "banner waving" chief they expected.

They praised McLaren for requesting, over Burrough's objections, a special grand jury to investigate the May 1977 shooting deaths of Alan Foreman and his girl friend Donna Shoemaker. After nearly a week of tense negotiations between Burroughs and McLaren, that request was finally rescinded by Ford.

"I think a lot of people gained a lot of respect for him," one officer said.

It was rare praise for McLaren. Most officers appear indifferent to the chief and regard one of this top assistants, Capt. William K. Stover, as the power to be reckoned with in the department.

Union head Jeunette said that among McLaren's successes had been his decision to give patrol officers more investigative powers and his work to get the police department an adequate budget.

"The biggest bummer of them all," however, was McLaren's push to establish team policing in Arlington, Jeunette said. Team policing is a program in which the police department is decentralized and the county divided into districts, with special departments in each district for different types of crimes.

Much of the bitterness also stems from McLaren's background, since he is not the "everyday, street guts policeman" who came up through the ranks, officers said.

Although Arlington is the largest department McLaren has served in, he began his career in Berkeley, Calif., in 1951. He also served as assistant chief in Dale City, Calif., and later organized a police force in Novato, Calif.