Fluidized bed granulators have been installed in pharmaceutical companies, and many trial studies to clarify the granulation mechanism have been reported over the past years. A granulation model was proposed in the previous study which was mainly concerned with the moisture content of powder in tapered-fluidized bed granulation. It was also pointed out that the viscosity of the binder liquid could be one of the important factors governing the granulation at a lower moisture content. In this study, the effects of the binder viscosity on particle agglomeration were evaluated for the purpose of explaining its contribution to particle agglomeration. Experimental results revealed that the initial agglomeration rate was determined by the binder viscosity. Based on these results, an agglomeration model was proposed taking account of the effects of the binder viscosity. Finally, it was shown that the value of the average granule size could be estimated by the moisture content and the binder concentration in the powder.