We report the size measurement of monodisperse aerosols generated from two electro-static classifiers. Sodium chloride aerosols, ranging in size from 0.003 to 0.2 μm, were produced. Tube-type and screen-type diffusion batteries were used to measure particle sizes. Aerosol penetration through both diffusion batteries was measured by a continuous flow, condensation nucleus counter. Our results showed that a more accurate voltage meter should be used to determine the applied voltage in the classifier. This is especially important for particles of smaller than 0.01 μm, where the applied voltage is smaller than 40 V. Measured sizes were in good agreement with those predicted by electric mobility for particles larger than 0.02 μm. When a condensation nucleus counter was used to measure penetration of particles smaller than 0. 02 μm, diffusion batteries gave larger apparent sizes than predicted. The explanations for this phenomenon are that ultrafine, classified aerosols have a large standard deviation because of diffusion mechanisms and the condensation nucleus counter (CNC) counting efficiency decreases with particle size. Recommendations for using electrostatic classifiers for particles of smaller than 20 nm are made.