Ultrasonic vibrations are used in many fields to reduce friction forces. In this paper, the reasons for the reduction of friction in solid surfaces are investigated by using the friction model of Dahl and the elastic-plastic contact model. Based on the theoretical model, four parameters: relative velocity, contact surface, distance between surfaces, and Young's modulus, are effective in the frictional force reduction. This study is validated using experimental tests. The results showed that the effects of oscillations of the relative velocity and changes of contact surfaces on the friction reduction are 51% and 12%, respectively. The minimum effect, among the factors, was related to Young's modulus with a value of 1%. The reason for the force reduction is the nonlinear behavior of the contact surface, contact stiffness, and the friction force functions. Fluctuations in their input parameters causes an asymmetric oscillation in the value of those functions. This feature changes the average value of these functions and therefore reduces the friction forces.