This paper describes the application of a conceptual rainfall runoff model to investigate the peak and monthly flows at the Sarisoo River Basin on the North West of Iran. The model was calibrated using measured stream flow data and then validated for three years. Calculations of level and time of peak flows are vital for designing structures downstream in the catchment areas. The simulated peak flows were occurring in the months of February in 2003, 2006 and 2007 with approximate values of 6.32, 9.35 and 6.13 m3s-1 respectively. After calibrating 9 NAM parameters using record data of daily rainfall, monthly evaporation and daily discharge in the period of 1th October 2003 to 31th March 2006 and validating the model daily discharges were calculated for 12 years. The outputs of the calibrated model are able to be used in the assessment of water resources management models like Mike Basin, WEAP… because they normally work based on monthly flows with a large time horizon. The results show that monthly averages of mean, maximum and minimum flows are about 10%, 2% and 33% less than daily computed Nash–Sutcliffe coefficients, all calculated over a period of 12 years. The optimum values of the 9 NAM parameters obtained during the calibration procedure are presented. The reliability of MIKE11 NAM was evaluated based on the Nash–Sutcliffe coefficient (R2), Root Mean Square Error (RMSE), peak flow (RMSE) and low flow (RMSE). The R2 obtained during this study is 0.74.