Abstract: CrMoV and NiCrMoV dissimilar steel welded joints were subjected to high cycle fatigue tests at room temperature and 400 ℃, fracture analysis was carried out on the failed specimens to study the high cycle fatigue failure mechanism of dissimilar steel welded joints. The results show that the S-N curves showed a continuous decrease, and there is no fatigue limit plateau observed during high cycle fatigue, while all the failure locations were located in the weld metal region, which was defined as the structurally weak zone with lower micro-hardness; with the increase of the high fatigue lifetime, the crack initiation mode was changed from the competition between surface and internal initiation to the dominated internal micro-defect initiation, the internal weld porosity micro-defects were the main source of crack initiation; The fatigue limits obtained by the Murakami model were dispersive and there was no apparent law between high cycle fatigue life. By introducing the Z parameter model, it was pointed out that the high cycle fatigue lifetime of welded joint was determined by the stress level, effective size and relative depth of micro-defects, and the Z parameter had a good linear relationship with the high cycle fatigue lifetime.