In the previous paper Yabuki and Matsumura, this issue , the critical impact velocity in the erosion of metallic materials by solid particles impact was experimentally determined. In this paper, the critical impact velocity was theoretically derived through analyzing the behavior of the material surface obliquely impacted by a spherical solid particle, where the material is strained tangentially to the surface in the direction of particle movement. As the extent of strain goes over the elastic limit, the impacted solid particle does skid over the ลฝ . surface, which brings about wear to the surface by cutting. The threshold strain was theoretically derived as a function of the impact velocity and the rotating velocities as well as the duration of particle surface contact. Those parameters which characterized the impact behavior were derived from the coefficient of friction and the rebounding coefficient, all of which were obtained from the mechanical properties of the target material and particle, and other factors concerning the particle. Consequently, the theoretical value of the critical impact velocity was given solely as a function of the mechanical properties of the target and the particle. The calculated critical impact velocity attained the lowest value at a low impact angle, and a good correlation was found between the critical velocity determined by experiment and that predicted by the theoretical equation.