Two field experiments were carried out at Mallawi requirements research station El-Minia Governorate, Egypt; Water Management Research Institute – National Water Research Center during 2012 and 2013 seasons. The aim of this investigation is to evaluate the effect of the different irrigation regimes and potassium fertilization rates on crop coefficient of potato. Also it evaluate and compare the potential evapotranspiration (ET_p) with actual water requirement under El-Minia Governorate conditions. The experiment included five treatments of irrigation regimes (A) and four treatments of potassium fertilization (B) with three replicates so that the experiment was arranged in a split plot design. The irrigation regime treatments were treationial irrigation (the farmers practes, 100%, 90%, 80% and 70% of field capcity). Potassium rates was applied in a form of potassium sulphate (48%K₂O) at rate 200 kg/fed. (b₁), 100 kg K₂O/fed. + potasine (Biofertilizer) at rate 6 liters/fed (b₂), 100 kg/fed. + potasine (Biofertilizer) at rat 6 liters/fed. + potassium foliar (36% K₂O) at rate 2 liters/fed. (b₃) and Potassium (biofertilizer) at rate 6 liters/fed (b₄). These results indicated that the first irrigation treatment where plants irrigated with conventinal irrigation had the highest value of actual consumptive use (daily and seasonal). This was due to the decrease in the value of tension moisture of the first treatment which led to increasing the water actual consumptive use. While, the fifth irrigation treatment for plants irrigated until 70% of field capacity had the lowest value of actual consumptive use (daily and seasonal). The application of Potassium fertilization caused a slight decrease in daily, monthly and seasonal  actual evapotranspiration(ET_a), in both seasons. Modified Penman and modified Blaney & Criddle gave high average values for potential evapotranspiration (ET_p) (63.24 and 56.50 cm/season) while radation method and pan method gave less average values (53.99 and 49.11 cm/season) for the two studied seasons respectively. The actual values of evapotranspiration were less than those computed by climatological equations. This is due to the estimated factors in these equations.                                        The average values of potential evapotranspiration (ET_p) for the two studied seasons, by modified Blaney & Criddle and radation method were the nearest values to general average (+1.41 and -3.09% respectively). While, the farthest values to general average were obtained by modified Penman and Pan method (+13.52 and -11.85% respectively). Kc average were 0.76, 0.72, 0.71, 0.70 and 0.68  for A₁, A₂, A₃, A₄ and A₅ under all sub-treatments respectively. Modified Blaney & Criddle was the nearest to the actual consumptive use. Therefore, recommended for calculating the potential evapotranspiration using modified Blaney & Criddle or radiation method for potato plants which grow under El–Minia conditions and other corresponding conditions.