Considering the increasing impacts of human on the enhancement of greenhouse gases and, hence, global warming, much attention has been paid to the studies on climate change in the developed countries during recent decades. Atmospheric carbon dioxide levels have been recorded continual increases since the 1950s, a phenomenon that may significantly alter the global and local climate characteristics, such as temperature and available water resources. Increases in carbon dioxide and other greenhouse gases in the atmosphere have caused global temperatures to rise by an average of 1°F over the past century. This global warming has resulted mainly from human activities such as the combustion of fossil fuels and deforestation. Global temperatures are expected to rise more this century as emissions of heat-trapping gases continue to mount. While the impacts of climate change will vary from region to region, it’s clear that almost every place on the planet will be affected. For assessing and detecting climatic changes, assessment of sensitive parameters in evapotrairation calculating can be suitable selection which affect climate condition and climate change. In Iran , as an arid and semi-arid country, around 93.5% of the total water resources has been used for agriculture purposes, therefore changes in climatic regimes may affect the agricultural water demands, because evapotrairation may be affected by the changes in meteorological variables (e.g., temperature, solar radiation, wind speed, and relative humidity). In this study were used fifty four-year monthly records of temperature, relative humidity, sunshine duration, wind speed, and precipitation depth from 1951 to 2005 in five climatologically different regions of Iran . The regions include Tehran , Esfahan, Shiraz , Tabriz , and Mashhad . The monthly evapotrairation were calculated by Penman-Monteith formula in these regions and were comprised with lysimeter data for estimating of Penman-Monteith precision and found it as a good result. A sensitivity analysis of meteorological variables was carried out using Penman-Monteith formula. Temperature, wind speed, and relative humidity were achieved as sensitive parameters