In membrane processes, fouling from ongoing deposition of particles is the major issue for membrane failure and thus many researches on fouling characteristics have been conducted. Most of the researches have been performed with monodisperse particle systems to clarify effects of particle size on the deposition and fouling mechanisms. However, particle systems in nature have a wide size distribution with polydispersity. In this study, we used an electrospray-scanning mobility particle sizer method, which is suitable for characterizing the concentration of each component in mixtures. A series of filtration tests for 0.03 tun rated Polyethersulfone membrane against mixtures of different sized polystyrene latex particles was performed. The results showed a significantly enhanced retention efficiency of 60 nm particles in mixtures with larger particles. This was attributed from the larger particles clogging large pores, so 60 nm particles could not penetrate. For the cases of mixtures of 60 + 150 nm and 100 + 150 nm, we found much more pressure drop increases compared to the summation of the pressure drop increase for each monodisperse case. In these mixture cases, smaller particles supposed to have a depth filtration mechanism formed the cake layers due to initially blocked larger pores.