In crossflow microfiltration, the tendency of particle deposition of polydisperse suspensions has been established experimentally and compared with that of monodisperse suspensions. The mass transfers of particles are different according to size in polydisperse suspensions. The most particles, which deposit to membrane surface without clogging pore in microfiltration, are much larger than 0.1 μm. Among these particles, smaller particles are easier to deposit than larger particles because of shear-induced diffusion and particle deposition depends on the size distribution of small particles. Effective particle diameter is introduced as a representative particle size which can reflect the diffusivity of each particle according to size and it describes the tendency of particle deposition very well in polydisperse suspensions. The effect of effective particle diameter is larger than that of feed concentration. The most important factor affecting particle deposition of polydisperse suspensions is effective particle diameter. The results of our research suggest that the effective particle diameter can be an important factor which can represent the potential for cake formation.