Abstract: In order to expand the research range of ultrasonic fatigue,the three-bending and non-symmetric tension-compression fatigue properties of 50 steel were tested and corresponding fatigue S-N curves were obtained.The effect of mean stress was studied.The mean stress shoued remarkable effect on fatigue strength of 50 steel under ultrasonic frequency loading.With increasing of mean stress,fatigue life decreased under the condition with the same stress ampitude.The model of Goodman describing the effect of mean stress in conventional fatigue can still be used to describe perfectly the relation between stress amplitude and mean stress at ultrasonic frequency loading.The model of Morrow that describes the effect of mean stress in conventional fatigue can still be used to describe the effect of mean tensile stress on fatigue behavior in LCF and HCF regime,but it can not describe the effect in ULCF regime at ultrasonic frequency loading.