Experimental and theoretical studies have been conducted about the influence of particles and dispersion medium behavior in the sedimentation bath on the measurement of 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 velocity of the downstream outside the tray and lessens the particle movement in the radial direction. Hence, the initial gradient of a sedimentation curve is larger than that of the theoretical line, and the total sedimentation mass agrees with the theoretical value. Since the velocity of the downstream outside the tray decreases with decreasing diameter of the tray, both the gradient and total mass come close to the theoretical value. It is found that the size distribution can be measured more accurately by using a detective tray whose diameter is smaller and peripheral wall is higher. Furthermore, it is suggested that the numerical simulation proposed in this paper can optimize dimensions of the sedimentation balance type particle size analyzer.