Abstract: Three post weld heat treatments（ 690 ℃×1 h,690 ℃×8 h,730 ℃×2 h） were conducted on the welded joint of Cr-Mo heat resistant steel. A combination of scanning electron microscope and microhardness tester was employed to characterize the microstructures and observe fracture surface of impact specimen,and then the mechanism of toughness variation at room temperature was revealed. It is shown that the welded metallographic microstructures of heat resistant steel welded joint are composed of martensite-austenite constituent and ferrite at smaller size. However,the microstructures of the welded joint after post heat treatment are made up of ferrite and carbide. The precipitation of carbide results in reducing the sub-grain size of ferrite,yield strength and microhardness,therefore,high toughness value of the welded joint obtained by post heat treatment（ 730 ℃×2 h） can be mainly attributed to the delayed occurrence of cleavage fracture.