This study on the party wall of a flocculation basin provides important ground to facilitate the inducement of uniformity in the rectangular sedimentation basin and to achieve an improvement in sedimentation efficiency. In the water treatment plant used for this study, perforated baffle type, square type, pillar type and downward rectangular type partitions have been applied. We evaluated the hydrodynamic behavior of several types of party walls in the flocculation basin by using computational fluid dynamics (CFD). The perforated baffle type demonstrates more effective output for uniform flow than the square type, and the third party wall of the flocculation process has the most influence of the three party walls for water flow distribution. To prevent sinking of the flocs formed between the third party wall and the final outlet wall, it is necessary to develop the third party wall into an actual final outlet wall or to modify it into a pillar baffle type. In the case of a drinking water treatment system that treats low density water, a perforated baffle is more efficient as the final outlet wall because a downward rectangular type may form a bottom flow, which may cause a reduction in the efficiency of the volume capacity.