Scattering angular function and responsibility of an optical particle counter are calculated by Using Mie scattering theory of spherical particle in this paper. The calculations indicate that when non-absorptive particles are measured, the 60?-scattering device is better than the 90°-scattering one. When absorptive particles are measured the measurement results of both devices are dependent on the refraction of the particles, so that it is possible to generate a considerable error. However, aftor the responsibilities of these devices were calculated, it is possible to correct the measnremont results upon refraction of the particles. This is important for size distribution of the atmosphorio aerosol.