The effect of Nb addition on the creep strength of high-Cr martensitic heat-resistant steel was investigated at temperatures ranging from 839 to 894 K. The short-term creep properties were improved by Nb addition due to the enhanced MX precipitation. However, Nb addition deteriorated the steel’s long-term stability resulting in an even shorter creep-rupture life than the original alloy. Onset of the accelerated degradation in Nb-containing steel was attributed to the gradual formation of Z-phase at the expense of M2N precipitates during creep exposure.