The purpose of this research is to calibrate and validate the HEC-HMS model as well as to simulate floods as a single event in Tajan basin. In this study, firstly the variables of precipitation and temperature were entered into the model, then the model was calibrated based on 3 single events and its parameters were calculated. Also, the parameters of curve number (CN) and impermeable surfaces were analyzed in GIS environment and finally based on another single event. The model was validated and its accuracy was estimated. The hydrographs generated from the scenarios of the adjusted model parameters were compared with the observed hydrographs of Kordkheil station. Calibration results showed that the simulated data has a high correlation with the observed data (R>0.9). Sensitivity analysis of the model was performed to determine the exact value of a sensitive parameter in exchange for changes of increase and decrease of parameters by 20%.The results of the sensitivity analysis of the model showed that the parameter of the curve number and the amount of initial losses are highly sensitive, so that in the range of -20 to +20% change of the parameter of the curve number and the initial losses, respectively, the sensitivity of the parameter from 7.15 to -2.33 and Varies from -0.79 to -0.73. Also, the results using correlation coefficient and -t Student test of model validation showed that the coefficient of explanation of the observed and simulated discharge is equal to 0.9. On the other hand, the model simulation results showed that the best objective function for estimating flood peak flow is Percent Error Peak, which the model predicted with a difference of 0.8% peak flow at Kordkheil station. Also, the objective function of the Percent Error Volume as the best estimator for estimated the flood volume with a difference of 0.