The fact that the fretting fatigue strength is significantly influenced by the relative slip amplitude was confirmed experimentally and reported in the previous paper. In the present paper, in order to make clear why the relative slip has a marked influence upon the fretting fatigue strength, the relation between the frictional force and the relative slip is investigated experimentally. As the result, the alternating stress amplitude, σfw1, which is the fatigue limit based upon the initation of fatigue cracks under the condition of fretting, could be expressed in the following equation as a function of relative slip, S, σfw1=σw1-2μp0{1-exp (-S/κ)}, where σw1 is the fatigue strength of plain specimen, μ is the coefficient of friction, and k is a constant.