The effect of boron on microstructure evolution and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3W-3Co-0.2V-0.05Nb steel at 650oC. Creep tests were carried out at 650oC for up to about 6 x 104 h. The addition of boron retards the onset of acceleration creep at low stress and long time conditions, which results in lower minimum creep rate and longer time to rupture. The addition of boron also retards the Ostwald ripening of M23C6 carbides near prior austenite grain boundaries (PAGBs) during creep. The retardation of the onset of acceleration creep results from the retardation of the recovery of martensitic microstructure near PAGBs by pinning effects due to fine M23C6 carbides. The main effect due to boron is considered to occupy vacancies near growing M23C6 carbides, which makes it difficult to accommodate local volume change around the growing carbides. This reduces the rate of Ostwald ripening of M23C6 carbides.