Lack of water which Iranians have faced it from the old times, it has created such a technical, cultural and socio-economical complicacies that even the scientific exploitation of these resources are affected, hence before any decision making, the hydrological dimensions of these should be explored. Using of empirical relation of the rainfall - runoff is known as one of the simplest way in calculating the outlet runoff volume of the basins. By the development of the computers and their calculating and logical decision abilities, researchers have developed more accurate and more efficient methods for different problems and phenomena. In this research rainfall- runoff process of Mahabad dam basin is simulated by the HEC-HMS conceptual model. The simulating process is done by using the SCS curve number and Initial and Constant Loss methods for calculating the loss rate, and Clark unit hydrograph and Snyder unit hydrograph for calculating the runoff rate, and their performance was evaluated using statistical criteria, Peak-weighted Root Mean Square Error (PRMSE) and correlation coefficient (R2). Reviewing the Simulation results indicate a better adaptation of the SCS Curve Number method in compare to Initial and Constant Loss method.