In Korea, the party walls of the flocculation basin consist generally of four parts, as three party walls and the outlet wall. The space between the third party wall and the outlet wall is therefore not the full extent of the flocculation basin and sedimentation basin. This space is theoretically totally unfounded. In order to practically apply the third party wall as an outlet wall, the volume capacity of the last third flocculation process needs to be determined on the ground that the perforated baffle (opening ratio 6%) is not ideal for the rectification effect. The buffer zone, according to the G value, is required in the last third flocculation process for a uniform flow at the outlet. Therefore, the objective of this study was to determine the size of the buffer zone in the last third flocculation process using CFD. Conclusions are made as follows: The difference of outflow rate percentage between the upper, middle and bottom part sections reduces according to the expansion of the third basin volume from 4.2 to 7.7 at each G value. We can suggest that the effluent percentage at the three sections is less affected by the G value than by the volume increase. For G values ranging from 11 to 16(1/s), the buffer zone needs to range from 10% to 20% compared with the last third volume by the velocity of standard deviation of 5 at the outlet. Also, when the velocity of standard deviation at the outlet is 2.5 or less, the buffer zone needs to range from 45% to 55%.