Almost 18 percent of Virginia's 10th graders failed the state's new competency tests in reading and mathematics, the State Education Department disclosed today.

State officials said almost 42 percent of black students and about 11 percent of whites failed the tests.

The 159-question test was given to about 71,000 10th grade students throughout the state last fall, and 11,861, or 17.8 percent of those without handicaps, failed both its parts. They will be given special remedial work and will have three more chances to take the exam before graduation, state education officials said. They must pass the test before they can graduate from high school.

Richard L. Boyer, an assistant state school superintendent who discussed the test scores at a press conference, said he was unsure why the failure rate for blacks was about four times greater than that for whites. But Boyer noted that blacks also have scored relatively low in other testing programs, such as the National Assessment of Educational Progress and the College Entrance Examination Board tests.

"The result could be somewhat related to socio-economic levels," Boyer said, "and the opportunities that students have not only in school but outside of school to learn some of these life-like skills that we tested."

Boyer said he thought there was no racial bias in the test questions and said they had been reviewed for fairness by several black educators.

After the press conference, Jack Gravely, executive director of the Virginia NAACP, said that his organization would study the results but he did not wish to comment on them now.

Boyer said a breakdown of the test results by race was supplied in response to public questions. "It just seems to be one of the areas that people want some information about so they can decide what effect the minimum competency tests have."

In the reading part of the exam, 32.8 percent of black students failed compared to 9 percent of whites and 20.1 percent of students of other races, principally Asians. In mathematics, the failure rate was 34.4 percent for blacks, 7.8 percent for whites and 9.7 percent for other races.

The rate for students failing either one or both parts of the test was 41.8 percent for blacks, 10.9 percent for whites and 21.9 percent for other races.

In Northern Virginia, about 20 percent of the 10th graders in Alexandria schools failed the tests, while only about seven percent failed in Fairfax County. Arlington was exempt from administering the statewide tests this year because it administered its own version earlier.

Throughout the state, 17.3 percent of the boys failed the reading part of the exam compared to 11.5 percent of the girls. In mathematics the failure rates were about even -- 13.9 percent for boys and 13.5 percent for girls.

Virginia's new statewide exam is part of a nationwide minimum competency testing movement that seeks to insure that all students have mastered basic skills needed to function in society before they can get a high school diploma.

Boyer said he expects that by spring of their senior year, less than 5 percent of all Virginia students will be denied a diploma because they failed the exam.