The three-dimensional motion of all particles in a rotating mixer with two baffles was calculated using the discrete particle simulation. This simulation dealt with the mixing of single-component spherical particles which fill the mixer half-way. The effect of the baffles on the particles’ motion and on particle mixing was studied. It was found that the mixing curve, which shows the relationship between the degree of mixing and time, agreed well qualitatively with previous experimental results. The present simulation predicts that there is an optimal baffle length/height ratio for particle mixing, and that the deformation pattern of the particle bed is affected by the baffles.