In highland watersheds, runoff generated by snow melting plays an important role in stream water supply. SRM (Snowmelt Runoff Model) is a hydrologic model which simulates and predicts daily flow in mountain watersheds dominated by snow melting process. The SRM is based on the degree-day procedure which, is a widely used method but does not consider physical factors. In the current research, the factor of radiation was added to the degree-day model to develop a simple energy balance equation. Daily average radiation was calculated by albedo, shortwave and longwave radiation, daily maximum and minimum temperature and relative humidity. The snow covered area (SCA) was obtained from daily MODIS images. The developed model was applied to the stream flow data of Karaj Basin located in northern Iran and the results revealed that the coefficient of determination of the observed and estimated data was 0.677 while the differences between estimated and observed volume of runoff was -5.58%. Therefore, the radiation based of SRM increased the coefficient of determination of estimated and simulated discharge about 9.3%.