The aim of this work is to reach a better understanding of the ultrasonic shot-peening process and, in particular, the evolution of the shot speed distribution. A simple 1D modelling of the interaction between the shots and the sonotrode is carried out. The impact is considered as inelastic with an energy absorption that depends on the speed of the shot. It is found that after about 10 interactions (» 1s) the speed distribution in the chamber follows a Maxwell-Boltzmann distribution, which is the distribution found in a perfect gas at equilibrium. The influence of various process parameters such as the sonotrode amplitude, the vibration frequency on the average speed and on the Almen intensity is studied.