Abstract: The dissimilar steel welding was conducted on G115 steel and T92 steel with overmatching and undermatching welding materials, respectively. The effects of overmatching and undermatching on the microstructure and mechanical properties of the two joints were investigated. The results show that the microstructures in weld and heat-affected zone of the two joints consisted of typical tempered martensite. The content of δ-ferrite near the fusion line at T92 steel side in the overmatching joint was higher than that in the undermatching joint. The average volume fractions of δ-ferrite were both less than 0.3%.The dilution of W, Co, Mo and Cu was observed in the fusion zone at T92 steel side in overmatching joint and at G115 steel side in undermatching joint. The microhardness profile on cross-section of the two joints was W-shaped, and the hardness fluctuation in the overmatching weld was relatively large. The tensile strength and yield strength at room temperature and at 650℃ of the undermatching joint were higher than those of the overmatching joint. The impact toughness in the weld of the two joints was the poorest. The impact fracture of the weld in overmatching joint showed a quasi-cleavage fracture mechanism and that of the weld in undermatching joint showed a microvoid coalescence/quasi-cleavage mixed fracture mechanism.