The aim of this research is to simplify the methodof getting Evapotranspiration Potentielle (ETP) which forms the limit of water needs for crops . This was using done some experimental Equation like Turc formula and Thornthwaite formula (Thornthwaite, 1948) compared with the modified Blaney_ Criddle formula ( BlaneyCriddle , 1950) which is considered the basic formula in this research . Also , this research aims to find a formula enables us to use (Pans evaporation) method (class A) as a relative evidence to the need of watering in Lattakia by comparing it with the results of Evapotranspiration Potentielle which are calculated by the modified Blaney _Criddle method depending on climate data from Lattakia for fifteen years (2001 – 2015) . The results of studying the correlation coefficient and how the experimental points spread around the linear regression curve show that Turc gave the best approximaltion result to the developed Blaney – Criddle formula with correlation coefficient (R=0.907). After Turc formula there is Thornthwaite formula with correlation coefficient (R=0.846) . On the other hand Pan class A method gave approximal results to the developed Blaney– Criddle formula with correlation coefficient (R=0.925) and with good distribution of the experimental points around the linear regressions straight. So, in this study experimental conditions it could beased on Turc formula and Pan class A method as the best approximation to the developed Blaney- Criddle formula to define the water requirement of the agriculture crops in Lattakia.