Experimental results on characteristics of viscosity η of unimodal and bimodal alumina/epoxy mixtures are shown and discussed based on theoretical consideration. Spherical alumina particles with nominal diameters of 35μm and 80μm, both of which had narrow dispersion of diameter distribution, were used as fine and coarse particles, respectively. η of the unimodal mixture increases remarkably with a volume packing fraction of particle ν_{p, and that Brinkman equation can be applied to estimate η from νp. On the other hand, η of the bimodal mixture depends not only on νp, but also on the ratio of coarse particles to fine ones. For larger νp, η of the bimodal mixture becomes smaller than that of the unimodal mixture. It is shown that an approximation procedure of η yields reasonable estimation for smaller νp, and underestimation for larger νp.}