Abstract: After gas nitriding three layers are formed on the surface of medium carbon railway axle steel from the exterior to interior,they are oxide layer,nitride layer and N-diffusion layer respectively.The effect of the three surface layers on the rotary bending fatigue properties of the steel was investigated in a very high cycle regime.Compared with the fatigue strength of untreated specimens,the strengths of nitrided specimens with the oxide layer,the specimens from which the oxide layer was removed and the specimens from which the nitride layer was removed greatly increased step by step.Nitrided specimens without the oxide layer fractured from the position between the compound layer and diffusion layer at the subsurface;while the ones without both the oxide and nitride layers fractured from the matrix.Through fractography and fracture analysis,the effect of the three surface layers on the fatigue properties was clarified in a very high cycle regime.