Because the encapsulants for semi-conductors contain high concerntration of silica as a filler,the viscosity behaviors of them largely depend on the content and the type of silicas. This study investigated the effects of the content and the average size of silica on the viscosity behaviors of the liquid encapsulant before curing. And such viscosity behaviors were fitted using a Mooney equation, which can consider the effects of content, shape, size and size distribution of fillers on the viscosity. Experimental results showed that both of intrinsic and relative viscosity depended on the average size of silica. Therefore, with the previous value of intrinsic viscosity, 2.5, which have been used independently for the size of fillers, the Mooney equation could not fit well the experimental data. More accurate fitting was made by using the experimentally determined intrinsic viscosities of each size of silicas. Moreover, the better predictions are expected to be possible by using the maximum packing fraction determined by considering the size distribution of silica.