A three-staged cascade virtual impactor was designed, fabricated, and used as aerosol classifier in a dust sensor module. The dust sensor module consisted of the impactor, three commercial dust sensors, and four pumps. For the design of the impactor, three cut-off diameters of 2.5 ㎛, 2.0 ㎛, and 1.5 ㎛ were selected. Then three nozzle widths were determined from computational fluid dynamics (CFD) simulation with the three designed cut-off diameters. Laboratory generated PM2.5 aerosols classified into each of three sizes, via the fabricated impactor, entered a dust sensor from which voltage signals were detected due to particle scattering by a laser diode in the sensor. The voltage signal data from the three sensors were converted to number concentrations of the dust particles utilizing correlation equations obtained from separately performed experiments. The number concentrations were in agreement with those obtained by aerodynamic particle sizer (APS). Keywords : Cascade virtual impactor, 3 stages, Dust sensor, Particle classification, PM2.5 measurement