In this study, chemical coagulation conditions for treating combined sewer overflow(CSO) occurred during rainy season were evaluated by jar tests with aluminum sulfate[$Al_2(SO_4)_3{cdot}17H_2O$] and ferric chloride[$FeCl_3{cdot}6H_2O$]. The raw domestic sewage sampled from the primary sedimentation tank at a local sewage treatment plant was filtered through $150{mu}m$ sieve before using. Point of zero charge(PZC) for various dose of aluminum sulfate occurred at pH 5.8-6.5, while for ferric chloride occurred at pH 5.3-6.0 in term of streaming current(SC) values. Charge neutralization ability of aluminum sulfate was bigger than that of ferric chloride. Optimum pH and dose of aluminum sulfate and ferric chloride were 6.2, 0.438mM and 5.8, 0.925mM, respectively. Removal efficiencies of TCOD, turbidity, SS and TP were 75, 97, 95, 96% with aluminum sulfate and 74, 96, 98, 99% with ferric chloride at their optimum coagulation conditions. More efficient removal of SS, TP and small particles was possible with ferric chloride at optimum coagulation conditions. Both SC values and COD removal started to increase where soluble phosphorus was completely removed.