We deal with a problem to determine experimentally as well as theoretically permeability of incompressible viscous flow through packed bed of bidisperse hard spheres in size. For the size ratios of large to small spheres λ =1.25 and 2, we set up bidisperse packing and measured porosity and permeability at various volumetric ratios of small to large spheres γ. Bidisperse packing shows lower porosity and permeability than monodisperse packing does. Variation of porosity as a function of γ does not match with that of permeability. A theoretical expression for predicting permeability of a viscous flow for packed bed of bidisperse packing is derived based on calculation of drag force acting on each sphere and its predictions are compared with the experimental data and those from some relations previously suggested. It is found that our theory shows better agreement with experimental results than the previous studies and is proved to be quite simple and accurate in estimating the permeability.