The effects of the reduced pressure on the minimum bubbling velocity and on the homogeneous fluidization region were investigated for three kinds of glass bead powders that belonged to group-A in Geldart's classification. The minimum bubbling velocity as well as the minimum fluidization velocity increased with a decrease in the pressure. It was found that the Reynolds number at the minimum bubbling velocity was related to the Knudsen number. The effects of the particle diameter and reduced pressure on the homogeneous fluidization region were experimentally examined. The homogeneous fluidization region became larger as the particle diameter decreased, and it became smaller as the pressure decreased.