Postmaster General William F. Bolger yesterday said he will ask for another rate hike next month approve an increase in first-class stamps from 15 cents to 18 cents, 2 cents less than he wanted.

The independent Postal Rate Commission last week rejected Bolger's proposal for a 20-cent first-class stamp, a 13-cent postcard and other rate rises in favor of lower increases. The commission recommended an 18-cent stamp and 12-cent postcard.

Bolger, who previously has said he would like to hold rate increases to three-year intervals, said, "We expect to file another rate case late this year," but he didn't say what rates he would request.

Bolger's comments before the Senate Governmental Affairs subcommittee on civil service, post office and general services were the first he has made on the rate commission's recommendations since the commission's decision last week.

The new rates recommended by the rate commission could take effect as early as mid-March and would result in higher parcel post costs and possibly higher prices for some magazines.

The postal governors have the final decision on the rates and are expected to take up the issue on Monday. At that time the governors can accept the rate commission's decision or reject it and return it to the rate commission for reconsideration.The governors can also approve the rates under protest, allowing them to charge the higher rates while seeking to overturn the recommendations in court.