A new line start sedimentation type sizing technique of aerosol particles using an ultramicroscope has been developed to determine the number density distribution of particle size in a direct manner. Aerosol sampled into a closed cell forms a thin layer at the top of the cell and then the particles settle toward the particle free space by gravity. The number of particles settling in the visual field of an ultramicroscope at a known depth of the cell is counted. This method has some advantages: (1) it directly gives the number density distribution of particle size; (2) it is suitable for aerosols having a high fluctuating size distribution with time; and (3) it permits a quick measurement in a few minutes, This sizing technique is useful for the size measurement of aerosol panicles of around 1 mu m in diameter and a high number concentration.