We determine the effective viscosity of suspensions with bidisperse particle size distribution by modifying an effective-medium theory that was proposed by Acrivos and Chang (1987) for monodisperse suspensions. The modified theory uses a simple model that captures some important effects of multi-particle hydrodynamic interactions. The modifications are described in detail in the present study. Estimations of effective viscosity by the modified theory are compared with the results of prior work for monodisperse and bidisperse suspensions. It is shown that the estimations agree very well with experimental or other calculated results up to approximately 0.45 of normalized particle volume fraction which is the ratio of volume faction to the maximum volume fraction of particles for bidisperse suspensions.