The Metro board, hoping to persuade Prince George's County commuters to leave their cars and ride the bus to rail stations, voted yesterday to raise parking fees at the New Carrollton Orange Line station and lower bus fares on some routes.

The county asked Metro to make the changes, to take effect Oct. 31, after watching the initial success of a similar program begun in March in Fairfax and Arlington counties, which also are struggling to keep up with the demand for Metro parking.

The $1 daily parking fee at the New Carrollton surface lots will rise to $1.50, and the $25 monthly fee for Metro's 400 spaces in the parking structure will go to $30. The fees for Amtrak's garage spaces will not change.

At the same time, one-way bus fares, now 80 cents, will be reduced to 50 cents on routes R11, R15, T12, T14, T16, T17 and T19. The usual rail-to-bus transfer discount will not apply to these fares.

Metro's 1,930 surface parking spaces at New Carrollton fill by 7:30 a.m. on weekdays, and buses serving the station are underused, the Metro staff reported to the board.

Similar problems in Virginia prompted Fairfax and Arlington to try comparable price changes to increase bus ridership to the Vienna, Dunn Loring, West and East Falls Church Orange Line stations and the Huntington Yellow Line station.

After two months, bus ridership had increased 30 percent overall, and by as much as 49 percent on some routes. The Huntington station parking lot filled later in the day, at midday compared with 8:30 a.m. before the program, but the parking lots at the four Orange Line stations continued to fill before the end of the morning rush hour.

Prince George's County also wants to increase bus ridership to relieve traffic congestion next year during construction along some major highways near the New Carrollton station, said Metro board member Richard J. Castaldi, also a member of the County Council.

The New Carrollton program will start as a six-month experiment, but Castaldi said he expects the changes to be permanent.

The Metro board, in other action:Approved Leroy R. Bailey Jr. as Metro's new assistant general manager for bus service. Bailey, 53, who began his transportation career in 1959 as a street car operator for the old D.C. Transit System Inc., succeeds Shirley A. DeLibero, who resigned in March to join the Dallas Area Regional Transit Authority. Bailey previously served as director of the largest Metrobus regional division. Approved John F. McElhenny as Metro's assistant general manager for design, construction and facilities maintenance.

McElhenny, 64, has served as acting chief of the department since the resignation in July of Forrest T. Gay III, who left to join a private firm. McElhenny previously served as Metro's director for construction. Learned that Metro's average weekday ridership increased 11 percent to 457,635 this spring, compared to last spring. More than half the increase was attributed to the opening of the Orange Line extension to Vienna in June 1986.