A method for simulating the motion of balls under the presence of a powder sample in a tumbling mill is proposed in this paper. The method is based on the particle element method and a coefficient of friction of colliding balls is the key to simulate the motion under powder filling. Four kinds of samples, i.e. glass beads, silica, aluminum hydroxide and kaolinite powders, were used for the milling. The movements of balls with and without a sample were observed visually by a video recorder and compared with the simulation results. The actual movement of balls in the mill is dependent on the kind of sample and the movement calculated by the present method can be simulated by choosing a suitable coefficient of friction for a given sample. The suitable coefficient of friction for the kaolinite sample is the highest value of all of the samples and is more than four times larger than that in grinding without sample. Suitable coefficients of friction for the samples are correlated with their angles of repose.