Combined sewer overflows (CSOs) are known for their overflow of untreated wastewater during storm events, therefore CSOs are composed of sewage and surface runoff wastewater, typically from urbanized areas. This study was performed to characterize the quantity and quality of the CSOs from urban watershed located in Hungduk-gu, Cheongju city and Dong-gu, Taejon city, respectively. Quantity and quality of CSOs were surveyed for two years and concentrations of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), total nitrogen (TN), total phosphorus (TP) and n-Hexane extracts were measured. The runoff loading rate of BOD, COD, SS, TN, TP and n-Hexane extracts were estimated to be 0.189㎏/ha/㎜, 0.692㎏/ha/㎜, 2.540㎏/ha/㎜, 0.032㎏/ha/㎜, 0.017㎏/ha/㎜, 0.771㎏/ha/㎜, respectively, during the wet weather. Based on the pollutant runoff unit load and washoff concept, ILLUDAS-POLL model (Illinois Urban Drainage Area System and pollutograph model) was developed. The Illinois Urban Drainage Area Simulator (ILLUDAS) has been modified for water quality model (named POLL) has been added to it. Parameter, coefficients adjustments for model calibration executions with continuous 5-minute rainfall record. From the results of model application, correlation coefficients between the measured and estimated concentrations were 0.53∼0.96 for BOD, 0.24∼0.91 for COD,0.32∼0.89 for SS, 0.55∼0.81 for TN, 0.47∼0.72 for TP, and 0.35∼0.95 for n-Hexane extracts respectively. It was concluded that ILLUDAS-POLL model is a useful tool in CSO`s pollutograph estimation with storm events for a small urban watershed.