The effects of particle size and weight fraction of fine powder (group C) upon the entrainment rate of bed particles (groups B + C or groups A + Q and fine powder were investigated in a fluidized bed and in a circulating fluidized bed. The entrainment rates and elutriation rate constants of the bed particles and fine powder were strongly affected by the particle size and the weight fraction of fine powder in the bed. A clear difference was observed in the dependence on the weight fraction of fine powder in both the entrainment rate of bed particles and the ratio of the elutriation rate constant of fine powder to that of coarse particles between the B-C and A-C binary particle systems. When fine powder of 6 mu m or smaller was added, the elutriation rate constant of fine powder decreased as the mean diameter of the bed particles decreased from 40 mu m. On the other hand, addition of group C powder of larger than 10 mu m to group A particles showed very good fluidization with the large entrainment rate of the bed particles.