Abstract: The gas kinetic theory of nanoparticle transport predicts that the collisions between gas molecules and a nanoparticle undergo a transition from specular to diffuse scattering when the particle diameter increases from molecular to micro-scale. We investigate the origin of diffuse scattering and verify the existence of the transition through molecular dynamics simulations. It is shown that diffuse scattering is the consequence of gas molecule absorption on the particle surface. Absorption results from the particle high ability of energy accommodation when the particle size is larger than a critical value.