Abstract: Corrosion fatigue crack propagation behaviors of ultranigh strength steel 30CrMnSiNi2A （b=1726.6MPa） within KImax707mV, SCE）, Kth is increased from 7.0 to 7.6MPa m . however, da/dN is also increased. SEM fractographic examination shows that a mixed type of intergranular and transgranular （mainly quasicleavage） failures with secondary cracks on the fracture surface is typical for all specimens tested in corrosion environments, while pure transgranular failure, is the characteristics of the micro-frac-tography of the. specimens tested in air. The proportion of intergranular failure on fracture surfaces for specimens tested in disilled water is only one half of that for specimens tested in 3.5%NaCl solution and increased with electrochemical potential applied. According to the crack propagation data and the results of fractographic examination, the mechanism of the wet corrosion fatigue failure of 30CrMnSiNi2A steel in the range of KImax