The Arlington County Board today will replace the only black on the county's school board with a former high school principal who was removed from that job for alleged incompetence and an inability to deal with minority students, county board members have said.

The selection of O.U. Johansen, 65, former head of Washington-Lee High School, has angered many blacks and community leaders, who say the appointment will end an eight-year tradition of having a black on the five-member school board.

"This is a racist thing," complained James Davis, a black community leader in Arlington, who says the county board's new Republican majority doesn't "want blacks in that position - at least that's the feeling of a lot of people in the black community."

But the three Republican-endorsed members of the county board denied in interviews yesterday that race was a factor in their decision not to reappoint Thomas L. Penn. to a second four-year term on the school board.

"Control of the school board by people who hold our philosophy is a key issue," said Walter L. Frankland Jr., vice chairman of the county board. ". . . With a black on the school board for the past eight years Arlington's school system has gone down."

County Board Chairman Dorothy T. Grotos said the change is being made for philisophical reasons. "It's the philosophy of education that counts, not the race," she said.

Grotos, Frankland, and Stephen H. Detwiler, all Republicans, said in separate interviews that they will vote for Johansen when the board meets today. It is the first major appointment the Republican majority has made since taking office in January and could become an issue in this year's races for two seats on the county board.

"The only course people have now is the ballot," Davis said. "People aren't satisfied and I would say that this will be an issue." Both Grotos and Frankland are seeking reelection this November.

The decision to remove Penn, a Democrat, comes at a time when blacks account for 14 percent of the county's school population and the percentage of nonwhite students in the system is rising sharply. Largely because of a recent influx of Asian and Hispanic students, nearly one out of three students in the county is a member of a minority group, according to county statistics.

"The county board is all white and now, for the first time in eight years, the school board will white," said John Robinson, director of the Martin Luther King Center located in Nauck, a predominantly black neighborhood in South Arlington.

"In a county like Arlington," Robinson said, "blacks should be on all important boards. There are qualified blacks and I don't want to see us go back to a situation like we had 20 years ago" when Arlington had a racially segregated school system.

In an interview yesterday Frankland rejected that argument. "I don't think that with a black on the school board, blacks will necessarily get the best break. Mr. Johansen is sensitive to minority issues, contrary to what some people have said."

"Arlington seems to have taken the brunt of the lower quality of education," said Frankland, who has been an outspoken critic of Arlington Superintendent Larry Cuban and what Frankland dalls his "inner-city educational philosophies."

Along with his Republican colleagues, Detwiler said that Johansen would restore a "balance" to a school board whose members were appointed by the county board's previous Democratic majority. "We have a fully intergrated school system," Detwiler said, "and I don't see this appointment as racical question so much as a philosophical one."

Democratic-backed county board member John W. Purdy said he opposes Johansen, whom he called "not and appropriate school board canidate." Purdy called Johansen's appointment, "obviously part of Walter's frequently stated purpose to get the superintendent."

In 1975, shortly after Cuban was appointed school superintendent, he removed Johansen as Washington-Lee principal, a post he had held for 14 years.

Cuban initially stated that Johansen was being transferred to an administrative post because he was nearing retirement age. Johansen, with the encouragement of parent groups, including one led by Frankland, then sued the school system for age discrimination.

In the midst of a rancorous court battle Cuban, in sworn testimony, said that he had concoted the story to spare Johansen's feelings. The real reason for the transfer, Cuban said was that Johansen was incompetent and "insensitive to minorities."

Both Johansen, who retired from the school system in January, and Cuban said yesterday that they believd they will be able to work together. "I'm not on the school board to carry out a vendetta," Johansen said.

"A seat for blacks on the school board is, perhaps, desirable," Johansen said, "but I'm not in on the politics of this. If (the Democrats) think it is so important maybe one of them should resign from the school aboard."