An experimental study on the tensile properties,impact toughness and FATT50 at room temperature and 565 ℃,especially the fracture toughness of base metal and weld of in a P91 main steam pipe has been carried out.The results show that tensile properties,impact toughness and FATT50 of weld are below base metal at room temperature and 565 ℃;and apparent crack initiation CTOD and conditional crack initiation CTOD of weld at 565 ℃ are higher than at room temperature.