Measuring the size distribution of fine particles (<5 mu m) in a few hours is difficult when using the sedimentation method due to a decrease in sedimentation rate. Herein, we discussed the validity of using a combination of the buoyancy weighing-bar method and the Rosin-Rammler equation to estimate the particle size distribution. When the cumulative mass oversize exceeds about 0.15, the Rosin-Rammler equation can be used to estimate the particle size distributions of suspended solids.