Abstract: The microstructures and mechanical properties of 12Cr1MoV steel welded joints before and after high temperature service were studied by optical microscopy, impact and tension test and micro-hardness test. The results show that after high temperature service, the microstructure of the welded joint consisted of not only ferrite and pearlite existed in the original structure but also granular precipitates. And the bainite was also found in the microstructures of heat affected zone and weld seam. The yield strength and tensile strength of the welded joint decreased from 455, 625 MPa before high temperature service to 430 MPa and 583 MPa respectively and the average impact energy decreased from 160 J to 124 J. Many cracks were observed at the impact fracture surface and obvious steps existed on them. The facture mode was plastic fracture. The hardness of base metal and weld seam after high temperature service were 190 HV and 265 HV respectively, and the hardness of weld seam met the technical requirement of the supplier while that of base metal exceeded badly.