The microstructures and mechanical properties of electron beam weldment BT25Y after 700℃/2h AC (AT) and 940℃/2h AC+600℃/8h AC (SAT) were investigated. The results indicated that the fusion zone microstructure of non heat treatment (NHT) specimen is mainly composed of α′ martensite phase and which show high microhardness and strengthen in comparison to base metal zone (BZ). For the specimens after AT, the α′ martensite in FZ transformed to fine α phase which shows similar microhardness regular pattern in comparison to NHT specimen. The fracture generated in BZ because of lower hardness and strengthens of BZ in comparison to FZ. Under this condition the specimen shows bad high temperature lasting strength. For the specimens after SAT, the α′ martensite in FZ transformed to α phase with rather big size and there is continuous or interrupted big size α precipitation bands along the original β grain boundaries. These precipitation bands in FZ lead to the brittle fracture of welded specimen and the fracture generated in FZ. Under this condition the specimen shows better high temperature lasting strength.