The particle size distributions measured by the optical microscope (OPM) were compared with those by the light scattering particle counter (PC) to validate the microscopic method for particle size distribution quantification. While the OPM concentrations increased with the PC concentrations, the OPM concentrations tended to be higher than those by the PC. To explain the difference between the two methods, we estimated their relationship based on the Mie scattering theory. The calculation roughly estimated the particle geometric diameters in theory were 1.7-2.0 times as large as the corresponding PC readouts. Using these theoretical factors, the size ranges of the PC were converted to match with the OPM measurements (PC*). Overall, the OPM concentrations were lower than the PC* concentrations. The advantage in the OPM method particularly for ambient aerosols lied in its accuracy of particle sizing although the counting efficiency might be lowered due to its intrinsic limitations such as inability of counting particles having the similar refractive index of the filter. (C) 2004 Elsevier Ltd. All rights reserved.