A novel particle classification system based on the integration of classification and diafiltration, both of which utilize cross-flow microfiltration under high flow rate, was successfully developed. In this system, particles could be classified into two fractions, one of which was smaller than pores of the membrane and collected as permeate and another collected as retentate. Diafiltration was then conducted to completely remove the smaller particles contained in the retentate to obtain another fraction consisting of larger particles. Importantly, classification and diaffitration should be conducted under cross-flow microfiltration at a high flow rate because the sufficiently large hydrodynamic lift force attributed to the cross-flow microfiltration operation keeps the membrane surface clean so it can act as a sieve. To show the effectiveness of our particle classification system, we demonstrated stable classification and continuous diafiltration for 26 h of bi-dispersed suspensions containing particles with diameters of 0.6 and 1.5 mu m and polydispersed suspensions with particle diameters of 0.1-3 mu m.