Abstract: The impaction of a single fly ash particle with a powdery layer consisting of uniform fine particles is numerically studied by means of the discrete element method. The influences of the incident velocity and the packed particle size on the microstructure of the powdery layer as well as the impact crater are systematically investigated. It is found out that with the increase of the impact velocity, the incident particle undergoes a transition from a complete sticking regime to a stable rebounding regime, where almost all the incident particles rebound after impaction. The critical incident velocity to reach the stable rebounding regime increases with the increase of the packed particle size. Furthermore, the evolution of the microstructure of the particle layer is evaluated quantitatively in terms of the local coordination number and void fraction, of which the variations increase with the decrease of the packed particle size, and increase quasi-linearly with the increase of the incident velocity. In addition, the normalized size of the impact crater is found to follow a 1/5 power law of the normalized impact energy. However, with the same incident velocity, the normalized crater size seems to be identical for different packed particle sizes.