The study deals with the effect of climate change impact on temporal trends, of the reference evapotranspiration and the crop water requirement of olive trees in the Sahel region of Tunisia, using the Penman-Montheith methodology described in the FAO Irrigation and Drainage Paper N°56.The temporal trend of the climatic parameters and the reference evapotranspiration of more than 33 years have been analysed using the Marm-Kedall trend test.The sensitivity of ETo has been studied in term of change of most important climatic parameters involved.