Most weather stations in Iran have been installed in arid areas, far from agricultural lands. Therefore, the data gathered are not valid for estimating reference evapotranspiration. The advantage of satellite imagery is the coverage of a wide surface region from where information regarding agricultural areas can be obtained. According to this issue, researchers have converted the Penman Monteith to a simple equation so that it only acts as a function of the surface temperature. The coefficients of this equation are functions of air aerodynamics and incidents of solar radiation and it has been demonstrated that, they are nearly constant as for a region in Argentina. Hence the reference evapotranspiration while using only surface temperatures was estimated with acceptable accuracy. The aim of this study was to evaluate the above mentioned simple equation for the Qazvin Irrigation System. A total of 231 images for a period of 3 years (2006- 2008) with no cloud cover of NOAA satellite images, covering the study area were made use of. The results of the study indicated that, the above mentioned coefficients were not constant for Gazvin irrigated area so as two equations were needed to estimate each coefficient from any day of the year were presented for making the needed estimations. The results showed that the simple equation is in demand of more calibration. The calibration was made and ET0 estimated with an R2 of 0.80 and a Root Mean Square Error (RMSE) of 0.47 mm d-1.