The performance of the Electrical Low-Pressure Impactor (ELPI) has been evaluated using monodisperse aerosols, ELPI is a near real-time size analyzer consisting of an aerosol charger and a cascade impactor, Particle cut sizes of the cascade impactor and the charging efficiency of the charger were determined experimentally in the designed range of the instrument, i.e. 0.03-10 mu m. The Stokes numbers of 50% particle cut sizes of all the impactor stages were found to vary from 0.421 to 0.483 with an average of 0.456 and standard deviation of 0.017. The collection efficiency curves were found to be steep with an average steepness (ratio of 70% and 30% collection efficiency) of 1.19. The charging efficiency of the charger was close to that specified by the manufacturer for particles smaller than 2 mu m in diameter. For larger particles, a deviation from the-manufacturer＇s specifications was observed. It results from the high particle loss in the charger because of the small-size of the charger, strong trapping electric field, and the perpendicular directions of the aerosol stream and the electric field in the charging zone. A comparison with SMPS for size distribution measurements was made in this study. The good agreement between the two measured size distributions shows the capability of the ELPI For near real-time particle size measurements.