A field experiment of drip-irrigated Sunflower (Helianthus annuus) was conducted at the Experimental Farm, Faculty of Agriculture (Saba-Basha), Alexandria University, Egypt during 2013 growing season to develop seasonal K_c values for drip irrigated sunflower. In this context the objectives were: 1. to analyze the ability of the FAO-56 single and dual crop coefficient models for assessment the regional evapotranspiration and water requirements, 2. to estimate an adequate water quantity needed for the sunflower. The sunflower variety Sakha 53 was cultivated at 28^th April and harvesting was done at 8 August, 2013. Seeds were sown at 4-5 seeds in each hill with a spacing of 0.3 m within each row and 0.6 spacing, then thinned to one plant after 2 weeks from sowing. After emergence, the plots were irrigated by the drip irrigation method. All field practices were done as usually recommended for sunflower cultivation. The irrigation treatments based on replenishment of soil water depletion according to reference evapotranspiration (ET₀). The irrigation treatments were; irrigation at 20, 40, 60, 80 and 100% of ET₀. The results indicated that seasonal sunflower evapotranspiration (mm) has higher value with field irrigation approach and the lower value was for standard FAO single approach. The seasonal evapotranspiration (single crop coefficient approach) was less than the seasonal evapotranspiration of dual crop coefficient approach. It appears that ETc estimation of sunflower crop is more accurate by dual crop coefficient approach than those produced by single crop coefficient approach because of using more parameters and taking the soil practices and crop characteristics in consideration. The basal crop coefficient values cannot be proposed for all climates and regions because of different climatic conditions and crop management practice under different regions. The present study recommended that for the present conditions and the same other conditions, the irrigation of sunflower crop must be done according to the dual crop coefficient approach because it is more accurate than single crop coefficient and close up to the field conditions.