The spape and site of baffle wall play a major role to uniformize flow pattern and to improve removal efficiency in rectangular sedimentation basin. This research has been studied on the mixing distance and time of jet flow, according to sizes, spaces and numbers of openings on baffle wall and shape of baffle wall in rectangular sedimentation basin, and has been measured flow pattern, detention time and depth of deposited particle from the inlet. As a result of this study, in the case 6% of opening area, the more small and narrow opening sizes are, the better flow pattern could be achived. The flow pattern and the deposit states of particles have been more improved when the cut -off plate is installed at the underflow of the baffle wall.