A new particle impact theory was proposed, which incorporated plastic deformation and elastic recovery of surface asperities into the overall energy balance of the impact process. Asperity heights were assumed to follow a truncated Gaussian distribution. The energy transfer associated with the deformation and recovery of asperities was derived based on the previous theoretical treatments of thermal contact resistance. It was found that even nanometer level roughness had significant effects on the rebound of micron size particles. The energy loss thus caused could explain the mysterious strong power-law dependence on particle size of critical velocity. Measurements of Wall et al. were compared with the theory. Good agreement had been reached between the theory and experiments for both coefficients of restitution and particle size dependence of critical velocity. (C) 2011 Elsevier Ltd. All rights reserved.