Model complexity is not sufficient for its accuracy. In practice, the models which are designed according to the nature of process and its applicability are more common. The main challenge is not to understand all the details about the process, but to make the problem simple and focus on the most important and effective factors. In this study, the WASMOD software is used to rainfall-runoff modeling. Despite of the simplicity of this model, the obtained results have good consistency with the observed values. It needs only basic meteorological data for a basin such as: precipitation and temperature, so it takes less time and cost for modeling. The model was run on a monthly time scale for two 15-year periods: calibration period (1975-1989) and verification period (1995-2009). The sensitivity analysis of the model’s parameters has performed based on Mean Absolute Error of the model relative to parameters’ changes. In the sensitivity analysis, the consistency between model results and observed runoff has been investigated. In general, it can be concluded that according to the accuracy level and the required input data, the WASMOD model has a good capability in simulating the monthly runoff. The Nash coefficient for model calibration and verification were obtained 0.62 and 0.55, respectively.