The relationship between mean residence time of fine particles and operating conditions in a Powder-Particle Fluidized Bed was studied using silica sand (d(p)=550 mu m) as coarse particles and, Al(OH)(3) (d(p)=1.4-15 mu m) and Al2(0)O(3) (d(p)=3 mu m) as fine particles. Fine particles were fed continuously to the bed and data were obtained under steady state. The mean residence time of fine particles increased with a decrease in gas velocity and fine particle size. When the relative humidity of the fluidization gas was 75 %, the mean residence time of fine particles (Al(OH)(3), d(p) = 3 mu m) was 3000 times as large as that of the fluidization gas. The effect of relative humidity depended strongly on the characteristics of charge with static electricity and wetting properties of the fine particles. There was a big difference in the mean residence time of each particle size when particle size distribution was wide. When the uniformity of the mean residence time of each particle size is needed, an adjustment of particle size distribution is required.