Abstract: Dissimilar metal welds between SA335P91 and 12Cr1MoV ferritic heat-resistant steels are widely applied in the main steam pipe and the reheater on boiler heating surface for the supercritical(SC) and ultra-supercritical(USC) thermal power units in China.The characteristics of microstructure and weldability of two steels were described, and the welding mechanism for dissimilar metal weld between two steels was investigated.Through theoretical analysis and experiments, the main factors in dissimilar metal welding that resulted in hot and cold cracks, decrease of toughness, embrittlement and type IV softening cracking tendency were determined.Corresponding technological methods and heat treatment procedures to prevent the above defects were proposed and solid joints were achieved.