The effect of boron on microstructure and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3WVNb steel with emphasis on the role of boron free from boron nitrides. Creep tests were carried out at 650oC for up to about 3 x 104 h, using specimens of 10 mm in gauge diameter and 50 mm in gauge length. The addition of boron in combination with no nitrogen addition effectively reduces the coarsening rate of M23C6 carbides by an enrichment of boron in M23C6 particles in the vicinity of prior austenite grain boundaries during creep at 650oC. This stabilizes martensitic microstructure during creep and retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. Excess addition of boron and nitrogen causes the formation of boron nitrides during normalizing at 1050-1150oC, which reduces dissolved boron and nitrogen. The dissolved boron enriches in M23C6 carbides, while the dissolved nitrogen causes the precipitation of fine MX carbonitrides. The variation of creep rates in transient region and of the onset time of acceleration creep with various combinations of boron and nitrogen contents can be explained by the dissolved boron and nitrogen concentrations after normalizing into account.