It was confirmed that it is possible to prepare particles of different shape by a dry impact blending method in our previous study. Here, the particle shape modification process is analyzed on the basis of a mechanical model. The particle shape is assumed to be spheroidal and is represented by an inverse of the elongation index. Then the shape becomes more spherical with treatment time and peripheral velocity of the rotor irrespective of particle size, but the shape index decreases with increasing powder concentration. It is, therefore, proposed that particle shape is to be deformed by impact force and to attain the limiting condition. The experimental results with stainless steel powder were well fitted with the present model,