In recent years, human activities induced increases in atmospheric carbon dioxide (CO2) which caused global warming and climate change. Climate change is anticipated to cause negative and adverse impacts on water systems throughout the world. Higher temperatures are expected to lead to a host of problems. These include melting snowpack, altering both the intensity and frequency of precipitation, increasing evapotranspiration and else. (Delghandi, 2016). Reference evapotranspiration (ETo) is a key hydrological variable quantifying a major water loss from catchments and basins, which can be used to calculate actual evapotranspiration (ETa), scheduled irrigation and prepare input data for hydrological models. The irrigation water requirement basically represents the difference between the crop water requirement and effective precipitation. The only factors affecting ETo are climatic parameters as water is abundantly available at the reference evapotranspiring surface (Allen et al., 1998). The first of climatic parameters is air temperature. As temperature increases, evapotranspiration also goes up. Some study conducted to indicate climate change impact on ET (e.g, Behmanesh et al., 2015; Sheidaeian et al., 2015; Babaeian and Kouhi, 2012). In this study, climate change impacts on reference evapotranspiration (ETo) and precipitation deficit (PD) were studied from 2010 to 2099 in Semnan region. The objective of this study was to examine the climate change impact on the ETo regarding the uncertainty of Atmosphere-Ocean General Circulation Models (AOGCM) and Greenhouse Gases Emission (GHG) scenarios.