The operational power of a co-axial double rotating cylinders mill with blades is estimated on the basis of the balls motion simulated by the Particle Element Method (PEM), The boundary condition on the rotating blades is introduced to simulate the balls motion. The estimated mill power is calculated from the balls motion in the divided working space to save simulating time. The actual and estimated power data tend to increase as rotational speed becomes high, while thp! remain at a low value and are independent of the speed of the inner cylinder when the speed of the outer cylinder is low. Both power data decrease at high rotational speed due to the rolling motion of the media balls, and the estimated power is in fairly good agreement with the actual one. It is confirmed that the boundary condition introduced in the present paper is suitable and applicable for estimating the operational power, as well as the balls motion in a rotational media mill with complex configuration in the working space.