Abstract: The effect of stress ratio on the fatigue crack growth behavior of J55 steel was studied by using SEM and electrohydraulic servo fatigue testing equipment.The results show that the fatigue crack growth rate decreased obviously with the increase of stress ratio R at the same crack growth length.The effect of stress ratio R on fatigue crack growth rate was not obvious at lower ΔKeff stage,but it was more and more remarkable with ΔKeff increasing.At the same Kmax stage,fatigue crack growth rate decreased with the increase of stress ratio R.Ductile fracture dimple gradually changed from parabolic shape to equiaxed shape at the high Kmax stage with increasing stress ratio.