For accurate estimation of unit hydrograph ordinates is very important in watershed management, many different methods have been developed for this purpose. Recently, Transfer Function (TF) model has attracted the attention of researchers, because it requires a very small number of parameters, omits negative values and also is robust against oscillations. After estimating the model parameters, the values of unit hydrograph obtain and then runoff would be estimated. For evaluating the performance of the TF model, it has applied on two case studies, namely Lighvan (in Iran) and Potomac (In North America) basins with two events in each basin and the obtained results were compared with the results of the traditional Least Square method. Root mean square error (RMSE) and coefficient of determination (R²) were used to evaluate the model performance. The R² values of the TF model for the estimated unit hydrograph of Lighvan basin and Potomac basin were 0.99 and 0.98, respectively. The results showed that the TF model with decreasing oscillations and omitting the produced negative values presented accurate results in estimating unit hydrograph and flood hydrograph ordinates and this method can be used in other watersheds