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One of the most important outcomes in the water balance equation for any natural area or water body is Evapotranspiration and it is also a crucial component of the hydrologic cycle. The relationship of temperature - evapotranspiration was obtained using the Thornthwaite method in Iraq by dividing the area into (7) sectors according to geographic latitude. Each sector has multi meteorological stations where (32) stations were used with different periods of records. A mathematical relationship was obtained between mean temperature and corrected potential evapotranspiration (PEc) with (97.45) to (99.84) coefficient of determination. Mean temperature has a symmetrical decreasing pattern from south-east towards north-west of Iraq affected by Mediterranean Sea climate conditions, while correcting potential evapotranspiration (PEc) has the opposite direction of increasing values due to a direct relationship with temperature.