Ain Sefra is one of the Algerian cities that had been experienced several devastating floods during the past 100 years. The purpose of this study is to simulate rainfall-runoff in the semi-arid region of Ain Sefra watershed through the employing of HEC-HMS model. In this paper, the frequency storm is used for the meteorological model, the SCS curve number is selected to calculate the loss rate and SCS unit hydrograph method have been applied to simulate the runoff rate. After calibration and validation, the simulated peak discharges were very closed with observed values.The Nash-Sutcliffe efficiency coefficient was 0.95 which indicated that the hydrological modeling results are satisfactory and accepted for simulation of rainfall-runoff. The peak discharges obtained for the 10, 50, 100 and 1000 years storms were respectively 425.8m3/s, 750.5m3/s, 904.3m3/s and 1328.3m3/s.Keywords: Ain Sefra, Floods, HEC-HMS, Frequency storm, Hydrological Modeling,Semi-arid