The influence of tank bottom shear stress on complete fluidization of spherical solid particles in a square surface agitation tank is Investigated. The shear stress on the tank bottom is measured by the electrochemical method, which utilizes the reduction of dissolved oxygen on point electrodes embedded in the tank bottom.The shear stress has a different value for each point electrode. It is found that complete fluidization of particles is attained when the shear stress all over the tank bottom exceeds the minimum shear stress required for the fluidization of particles, which is dependent on the properties of the particle and surrounding liquid in the tank. The empirical equation, which shows the relationship between the minimum shear stress required for the fluidization of particle and the properties of particle and liquid in the tank, is obtained. By use of this equation, the relationship between the minimum impeller speed required for complete fluidization and experimental conditions is further discussed.