Fretting fatigue tests were carried out using S20C steel to investigate the effects of relative slip amplitude and contact pressure between the specimen and the contact pad on fretting fatigue strength. The fretting fatigue life was reduced with an increase in relative slip amplitude, and it came to a constant value beyond a certain value of relative slip amplitude (12-13μm). With an increase in contact pressure, the fretting fatigue life was reduced. The reason why the fretting fatigue life is reduced with increasing relative slip amplitude and contact pressure may be as follows. An increase in relative slip amplitude or contact pressure produces an increase in frictional force which acts on the contact surface of specimen. Since the growth rate of fretting fatigue crack is influenced strongly by the frictional force, the increase in frictional force produces the acceleration of the crack growth rate and consequently gives a lower fatigue life. Since the frictional force comes to a constant value beyond the certain relative slip amplitude, a constant fatigue life is observed in this region of relative slip amplitude. This speculation was confirmed by the prediction of fretting fatigue life based on the elastic-plastic fracture mechanics, where the frictional force was taken into account.