Abstract: The effect of long-time aging at 650 ℃ on microstructure and high temperature strength at 650 ℃ of the 12Cr-4W-3Co heat-resistant steel was studied by XRD, SEM and TEM. The results show that the precipitates were M23C6 and Laves phase when the tested steel aged at 650 ℃ for 0-5 000 h, the yield strength at high temperatures gradually decreased. The reduction of the yield strength was mainly resulted from the martensite lath recovery during 0-100 h. The yield strength was basically unchanged for the increase of M23C6 and Laves phases precipitation and the decrease of martensite lath recovery during 100-1 000 h. The growth of M23C6 and Laves phases and recovery of the martensite lath led to the decrease of yield strength from 1 000 h to 3 400 h. During 3 400-5 000 h, the lath recovery stopped, and the yield strength basically unchanged for some precipitation and growth of M23C6 and Laves phases.