Electron beam（ EB） welding was applied to a Fe-Cr-Ni-Mo high-strength steel. Effect of tempering temperature on the microstructure and mechanical properties of welded joint was investigated by means of OM,SEM and TEM. The results show that the welded metal（ WM） consists of coarse lath martensite and there are finer martensite and a small amount of carbides in the heat affected zone（HAZ）. After high temperature tempering,large amounts of carbides precipitate in both WM and HAZ. Hardness tests show that the hardness of WM and HAZ are 560 HV0. 5 and 530 HV0. 5 respectively and tempering significantly lowers the hardness of the WM and HAZ,which is slightly higher than that of the base metal（415 HV0. 5）. Therefore,all the tensile samples of welded joint are broken in the base metal. The impact tests show that the impact absorbed energy of WM and tempered WM at 600 ℃ are 48 J and 94 J respectively,which indicates that tempering can significantly improve the toughness of WM.
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