The effects of gas humidity on fluidization were studied by measuring torque for stirring in the gas-solid stirred fluidized bed. For particles fluidized smoothly, such as glass beads or CaCO3 particles of large diameter, the torque for stirring was little affected by gas humidity, while for polymer particles or CaCO3 particles of small diameter their fluidization were affected by gas humidity. The torque for stirring of CaCO3 increased with gas humidity because of the increase in adhesive force caused by adsorptive water on the particle surface. For polymer particles, the torque decreased with gas humidity because the electrostatic force originating from the surface charge of particles was inhibited by adsorptive water. However, with further increase of gas humidity the torque increased, because of the adhesive force due to adsorptive water. The shear stress of fluidizing or static particles on acrylic or steel pipe was measured and the effect of gas humidity on them was investigated. © 1989, The Society of Chemical Engineers, Japan. All rights reserved.