This study is to evaluate control effects of separation wall by surveying water quality and sewer overflows during dry and wet periods in combined sewer and separated sewer systems. Ravine water from the combined Seokgyo outfall with the separation wall was separated about four times larger than sewage flow during dry periods. The water quality of the combined Seokgyo outfall with separation wall during dry periods is flow weighed average T-N 22mg/L, T-P 2.4mg/L, the combined Chenseokgyo outfall without the separation wall is average T-N 21mg/L, T-P 2.6mg/L, and the separated Pyoungsong center outfall is average T-N 12mg/L, T-P 1.0mg/L. The T-N, T-P concentration in separated outfall make form about 44%, 42% of the combined outfall, and this means the separated outfall (i.e. storm sewer) is polluted by inflow of sewage. The overflow load of the separated outfall is ten times higher than the combined outfall and its overflow load per rainfall is three times than combined outfall during the wet periods. Therefore, the control plan of overflow load is required in storm sewer. The control effects of the overflow load increased 100% by setting the separation wall in the combined sewer, and showed 44% increase without the separation wall in separated sewer, but forecasted over than 82% increase of effects with the separation wall.