Estimation of urban runoff peak and volume is a fundamental step in determining the transferring capacity of urban drainage systems. The main aim of this study was to present an application of the Storm Water Management Model (SWMM) in order to estimate urban flooding of a semi-arid area (Zanjan city in the northwest of Iran). The performance of an urban drainage system in the study area was also investigated. According to the results, SWMM is an effective tool for urban flood estimation in a semi-arid area. In this study, urban peak flow was simulated via a calibrated model with acceptable accuracy. Based on the results of the model simulation, the capacity of the main canals in the study area is sufficient for peak runoff transferring for a design storm with 50 year return periods, without retrofitting. Whereas, based on local observation and model results, localized and surface flooding can be observed in some urban areas.