Based on the particle morphology of beryllium (Be) powders produced by impact attrition, a physics model has been established for describing the particle size distribution, which has only one parameter with the definite physical meaning. Two important internal mechanisms are considered: firstly, the distribution of existing state of particles with different original kinetic energy should obey the Maxwell-Boltzmann distribution after impacting, which means that big particles are unstable for their being at higher energy level, therefore they are easy to be fractured into smaller pieces in impact attrition process, and this influencing factor can be described as the negative exponential function of particles size; secondly, the tendency to remain low surface energy requires that Be powders should keep big volume as much as possible, and this influencing factor is defined as the cube function of particles size. The actual size distribution of Be particles is resulted from the competition between these two factors. Calculating result from the model is in good agreement with data from measurement