Abstarct: Due to changes in earth's climate regimes, climate change is affecting various operating systems such as water resources, agriculture and the environment. So it could be one of the most important human challenges in the upcoming decades. In the present thesis, the effect of climate change on Talar watershed runoff was investigated. First, the surface runoff was simulated using a calibrated hydrological model, SWAT. The SUFI-2 algorithm in SWAT-CUP model was applied to calibrate SWAT’s parameters. For this aim, two objective functions R2 and Nash - Sutcliffe were used and their values were equaled to 0.626 and 0.61 for calibration and 0.99 and 0.88 for validation periods, respectively. Then downscaled climate data from a global GCM model, CGCM3, were fed to calibrated SWAT model from 2064-2046 and 2100-2081 time periods to simulate future surface runoff of the study area. Three emission scenarios namely, A2, B1 and A1B were used in the present study. Results showed that temperature in winters of 2064-2046 and 2081-2100 periods would be increased up to 4 ° C and 7 to 9 Co, respectively. Also, the average annual discharge at Talar station would be decreased from 1 to 2.8 cms for the future time periods under three climate change scenarios. These changes reflect the importance of appropriate management of water resources in the coming years at Talar watershed.