Abstract: The fatigue behavior of Low alloy steel in air and corrosion medium was studied by the fatigue tests. The S-N curves were obtained and the observations of flat surfaces and fatigue fractographs have been made. The results showed that the fatigue strength of Low alloy steel in corrosion medium was much lower than that in air. Fatigue cracks in air were initiated from the matrix of surface, while most of fatigue cracks in corrosion medium were initiated from pits. The typical feature of the fatigue crack propagation region in air was fatigue striations, while that in corrosion medium was intergranular crack characteristic. The fatigue limit in air could be predicted by empirical equation. The predicted fatigue limit was agreeable to the experimental fatigue limit.