CBS Entertainment President B. Donald (Bud) Grant yesterday announced his resignation after 15 years at the network.

He immediately announced the formation of Bud Grant Productions, which has already received a commitment from CBS for program development. In addition, he will serve as a consultant to the network.

With his departure, Kim LeMasters, vice president of programs, will take over the entertainment division on a trial basis, reporting directly to CBS Broadcast Group President Gene F. Jankowski.

Grant has been president of the division since 1980 and in recent years has seen NBC take over as the leader in prime time.

So far, the 1987-88 entertainment schedule he and LeMasters put together last May has not only failed to make a dent in the NBC lead but, following ABC's recent success with its seven World Series games, is now running a close third behind its two rivals.

Jankowski flew to the West Coast Thursday to confer with Grant and LeMasters, reportedly on direct orders from CBS Inc. Chief Executive Officer Laurence Tisch. CBS sources yesterday said Tisch had wanted a change at the top of the Entertainment division in January but Jankowski was able to win more time for Grant.