A simple mathematical model is proposed and optimum operating conditions by the help of the model parameters are defined. The impulse response (residence time distribution) of the crushed grain, product quality and the total energy consumption under various operation conditions (number of hammer rows, number of hammers in a row, mesh size of screen, velocity and output of the mill were used as variables) was studied on a model device of a hammer mill. The radiotracer was used for the experimental verification of the model and for the determination of a holdup (the magnitude of the rotating grist layer) on the pilot scale and partly also on the full-scale mills. The impulse responses were influenced by transport processes behind the screen. This effect had to be taken into account during the analysis of experimental data. The method of a non-linear regression analysis was applied for the evaluation of parameters of the mathematical model.