The thermal stability of the microstructure, in particular that of the precipitates, has been known to be a critical factor for the creep strength of 9–12Cr martensitic heat-resistant steels. In the present study, four martensitic heat-resistant steels were designed to have different precipitate types and morphologies. Steel NS1 (nitride strengthened, NS) has a low density of fine nitrides and Steel NS2 is strengthened by a high density of fine nitrides. Steel CNS (carbide and nitride strengthened, CNS) contains large size carbides and fine carbonitrides and Steel CS (carbide strengthened, CS) mainly possesses large size carbides. Both aging and creep tests at 650 °C were employed to evaluate the stability of the microstructure and properties of the steels during high-temperature exposure as a function of the precipitate morphology. It was found that Steel NS1 and Steel NS2 displayed a good creep performance, but they already recrystallized after a relatively short-term aging treatment. On the other hand, Steel CNS and Steel CS gave poor creep strength, but they could still maintain the tempered martensitic microstructure during long-term aging. The relation between microstructural stability and precipitate morphology/density was linked to the dislocation-precipitate interaction.