Prince George's County ninth and 11th grade students did better last year than in the past on state reading competence tests, but seventh graders showed no similar improvements.

Test results released yesterday showed that 74 percent of the ninth graders in the county's public schools achieved "competent" scores on the Maryland Function Reading Tests last fall as compared to 72 percent in 1977.

Eighty-eight percent of the county's 11th graders passed the basic reading test, an increase of three percent over the 1977 figure of 86 percent.

The county's seventh graders, however, made no improvement over the 1977 scores. As in 1977, 70 percent of the class demonstrated reading competence.

The reading competence test has been administered statewide since 1975 as part of the Maryland Accountability Program. The test requires a basic reading level needed to understand grocery advertisements, application forms and newspaper announcements. To pass, a student must answer correctly at lease 80 percent of the questions.

State officials said yesterday they have not yet calculated comparative statewide averages. In the four years since the test began, however, Prince George's scores have been several percentage points below the state average.

Last year's ninth grade class is the first to be affected by new graduation requirements that specity that all students must pass the reading competence test in order to receive a Maryland high school diploma.

Victor Rice, supervisor of evaluation and research for the county school system, said yesterday that ninth graders who failed last fall's test were retested in May and the second test increased to almost 85 percent the number who demonstrated competent reading skills.

Ninth graders who failed the May test will have three more changes to pass before their 1982 graduation date, he said.