In order to explore the effect of grain size on fatigue life, a formula of fatig ue life for polycrystalline metals,which depends on the grain size and cyclic st rain amplitude, is derived based on the non-equilibrium statistical theory of fa tigue fracture and the model of interface energy. Subsequently, the fatigue life s of pure copper and 70-30 brass with different grain sizes are calculated accor ding to the formula which is achieved in this paper, and the results are in agre ement with the experimental results.