Experimental and theoretical studies have been conducted about the influence of the particles and dispersion medium behavior in the sedimentation bath on measuring the particle size distribution with the sedimentation balance. It is found that an increase in the height of the peripheral wall of a detective tray raises the downstream velocity outside the tray and lessens the particle movement in radial direction. Hence, the initial rate of increase in a sedimentation curve is larger than that of the theoretical line, the total sedimentation mass agrees with the theoretical value. Since the downstream velocity along the inner wall of the sedimentation bath decreases with decreasing the diameter of the tray, both the initial rate and the total mass come close to the theoretical values. It is found that the size distribution can be measured more accurately with using the detective tray whose diameter is smaller and peripheral wall is higher. As particle size decreases, the error caused by the flow of the dispersion medium, that is, the error in cumulative undersize in the fine size range increases. This is because smaller particles are affected strongly by the flow of the dispersion medium. Furthermore, it is suggested that the numerical simulation proposed in this paper can optimize dimensions of the sedimentation balance type particle size analyzer.