In this paper, the characteristics of the flow and the movement of the floc were analyzed through the experiments in the sedimentation basin using the redioisotope. Tc-99m radioisotope was used for the experiments for the characteristics of the flow, and the bentonite absorbing Tc-99m radioisotope was used in the experiments for the movement of the floc. Through the experiments of the flow, it was found that the velocities of flow in the sedimentation basin were different depending upon the position and the depth. The distance of the moving trajectories of the floc is increased by increasing the discharge. By increasing the discharge, the settling point is farther from the inlet, and the turbidity removal efficiencies in the sedimentation basin are decreased. The moving velocities of the floc and the density are changed in the different depths in the sedimentation basin.