In this study, the microstructural behavior of two kinds of 9-12% Cr steels (specimen A and B) used for power plants with and without W, Co, B which were subjected to aging during various time and temperature were investigated by electron microscope and related analytical techniques. The results indicate that there are four kinds of precipitates in these materials, i.e., M23C6, Nb-rich and V-rich MX, W-rich and Mo-rich Laves phase, and Z-phase. Upon aging, the area fraction of M23C6 increased whereas that of Laves phases decreased despite an increase in size. The area fraction of W-rich Laves phase was much higher than that of Mo-rich Laves phase, indicating that W addition compared to that of Mo addition is more powerful in the formation of Laves phase precipitation (specimen A). Specimen A aged at 700°C exhibited the formation of Z-phase. The formation of Z-phase is known to have a tendency of dissolving MX precipitates in similar alloys. The high activity of Cr at 700°C is believed to be the cause of Z-phase formation.