Particle size analysis based on single particle observation in a sedimentation cell is investigated. The settling chamber consists of a 3 mm cylindrical channel in the centre of a copper block. The particles are illuminated with parallel laser light and examined by an ultramicroscope technique. Experiments are carried out with polystyrene latex spheres and a condensation aerosol of di 2 ethylhexyl sebacate (DES) in the size range between 0.3 and 5 μm. If, for particles of uniform size, the arithmetic mean value of the settling times is used, the Brownian motion has no influence on the size evaluation. For particles above ca. 1.5 μm, not only the median diameter, but the whole size distribution of the aerosol can be determined by the sedimentation method. The diameters derived from the settling velocity agree within a few per cent with results obtained by electron microscopy or optical size spectrometry. To achieve this accuracy about 100 single particles have to be counted for each size evaluation. The A value of the slip correction of polystyrene spheres is found to be: A = 1.34 ± 0.011.