Circulation of The Washington Star declined while circulation of The Washington Post increased during the six months ended March 31, according to data released by the Audit Bureau of Circulations.

The Star's Monday-Friday average circulation for the recent six months was 375,583, down more than 15,000 or 3.9 per cent from the same period a year earlier.Star Sunday circulation decreased by 7,774 or 2 per cent to 374,251.

Washington Post average daily circulation for the recent six months was 555,030, up 40,181 or 7.8 per cent from the same period ended March 31, 1976. Post Sunday circulation rose 47,435 to 766,241, a gain of 6.6 per cent.

According to ABC data, the Post had the largest gain in weekday circulation for the six-month period among the nation's papers, trailed by the aan Francisco Chronicle (up 26,906) and The New York Times (up 25,428).

A spokesman for Star chairman Joe L. Allbritton said last night his newspaper "looks at two-year figures as more comparable" because of a strike against The Post in the six months ended March 31, 1976, which added to Star circulation and reduced Post circulation temporarily.

The figures for March 31, 1975, show the the Star's daily circulation has increased 23,415 daily and 5,757 on Sundays during the past two years, said the Star spokesman. The Post noted that its year-to-year gains surpassed any since acquisition of the Times Herald more than two decades ago.

In another development yesterday, Allbritton's York County Coast Star, in Kennebunk, Me., announced publication of a new weekly in Sanford, Me., starting June 15. The Associated Press said the new Sanford Star will fill a vacuum created by the merger of the thrice-weekly Sanford Tribune with the daily Biddeford-Saco Journal.