Abstract: The microstructure evolution of T91-12Cr1MoV dissimilar steel welded joints treated by different aging was studied. The results show that there is obvious deterioration of 12Cr1MoV steel during aging, and the binding force of grain boundary decreases; at the side of T91 steel, lath martensite recovers after aging for 10000 h, and carbides gather and precipitate at grain boundary. The change rule of hardness near the fusion line is consistent with that of carbon migration. The hardness of the base metal region also changes with aging time.