Abstract: Effects of long-term aging at 630 ℃ on the microstructure and mechanical properties of G112 steel were investigated using SEM, impact testing machine and tensile testing machine. The precipitates and their contents in G112 steel for different aging times were studied using XRD and BSE of scanning electron microscope. The results show that there were Laves, M23C6, M(C,N) phases in the test steel aged at 630 ℃. As the aging time increased, the amount of precipitates increased. The growth rate and precipitates′ amount of the Laves phase were all larger than those of the M23C6 phase, and the average size of the Laves was smaller than that of the M23C6 phase, so the Laves phase had better strengthening effect than the M23C6 phase. During the aging process, the high temperature strength of the G112 steel aged at 630 ℃ for 3 000 h decreased by about 12%, impact toughness at room temperature decreased by about 52.2%. The change of mechanical properties of the test steel was the result of martensite lath recovery and precipitation strengthening.