Aiming at stable automatic operation of a fluidized bed granulator, three factors namely, moisture content, fluidization activity and input thermal energy were selected as principal ones affecting the granule diameter. The techniques of in-line measurement of these factors and their feed-back control were examined. The amplitude level of the stress wave generated by the fluidized particles was measured and applied for estimating fluidization activity. The new attempt quantified well particle properties and fluidization condition change. A fuzzy control system was established by the use of the production rules, determined by considering the degree of influence of the three factors having a strong interaction on granule quality. The ideal pattern of moisture content change in stable fluidization condition, which produces granules having narrow size distributions, was successfully obtained by using the established system, for even difficult granulation conditions by usual control methods.