During the head-on particle collision, the adhesion plays a more important role as theparticle size decreases to micro size; the increasing surface effect makes the particle prefer to aggregate. While on the other hand, as the impact velocity increases, particles prefer to separate because of the larger elastic repulsive interaction. Another factor, which cannot be ignored during the impact of metal microparticles, is the dislocation plasticity which shows the rate and size effect. In this work, taking nano-plasticity behavior into account, our molecular simu-lations revealed two critical impact velocities for the transition of particle collision from separation to aggre-gation, and these two velocities are quantified by the analytical models proposed in this study. The low critical velocity for particle aggregation is dominated by adhesion, while in contrast, the high critical velocity for ag-gregation is dominated by dislocation plasticity, where the dislocation density in the particle after the collision is proportional to the impact velocity. With these findings, an analytical model was proposed to determine the critical particle size, below which no separation will be found whatever the impact velocity is. And this critical size is proportional to the ratio of surface energy to stacking fault energy.