Description: The effect of antecedent precipitation index of Soil Conservation Service-Curve Number (SCS-CN) method (SCS, 1956) on the precipitation-runoff relationship was examined. Using long-term measured rainfall data from Istanbul-Catalca Damlica Creek Watershed (ICDC W) located in a semi-arid region, calculated runoff values under both taking 5-day antecedent precipitation amount into account and not taking it into account conditions were compared. There were not any statistically significant differences between the calculated runoff values under both conditions. However, when the calculated runoff values under both conditions were compared with the directly measured runoff values, calculated values were found to be significantly larger, up to 7 folds, than the directly measured ones. Therefore, SCS-CN method was criticised in terms of over-sizing hydraulic structures and increasing the cost.