The invention relates to a method and an apparatus for determining the size or the size distribution of fine particles. A single particle or a group of particles are shone by a parallel polarized beam of a single wave length and the scattered intensity on the plane of polarization of the incident beam is measured by a photodetecting array. For a single particle the size is determined from the peak scattering angle at which the profile of the product of the scattered intensity and the scattering angle has the peak. On the other hand, for a group of particles, the size distribution is determined from the angular variation of the scattered intensity or the profile of the product of the scattered intensity and the scattering angle measured on the plane of polarization of the incident beam. For fine particles mixed with large particles the scattered intensities on the plane at a right angle to the plane of polarization measured by another photodetecting array is used to improve the accuracy of size determination of the fine particles.