Increasing competition on water resources between the agriculture sector and other sectors requires new irrigation regimes to perform relevant levels of production in semi-arid regions. Field experiment was conducted during two successive seasons at the Experimental Farm of EL-Quassassin, Ismailia Governorate, Egypt. This experiment was prepared to assess the effects of different irrigation intervals, potassium phthalate and their combinations on the growth, number of umbels, fruit yield,water use efficiency, and volatile oil production of fennel plant during two successive winter seasons 2016/2017 and 2017/2018, respectively. Three irrigation treatments, main factor, as follow: I1(every three days, which is the common in this regionwith 1150 m3/ha), I2 (every five dayswith 766 m3/ha), and I3 (every seven dayswith 383 m3/ha), and the potassium phthalate as subsidiary factor (0, 400,500, and 600 ppm) were applied. The results showed that, the highest vegetative growth parameters were related to the potassium phthalate at the rate of 600 ppmand irrigation intervals 3 days.The highest values of fruit yield, as well asthe volatile oil production were significantly affected by the potassium phthalate and irrigation frequency in both seasons. Nevertheless, the concentrations of the macro and micro-nutrients in fruits revealed an increasing trend with increasing the irrigation intervals.In addition, the lower the amount of water use, the higher the water use efficiency.The results of GLC analysis of fennel volatile oil indicated that methyl chavicol as the main component varied between (63.02 and 80.82%). The treatment of potassium phthalate at 600 ppm with irrigation every 3days resulted in the lowest methyl chavicol percentage.Thus, the proper irrigation interval increases the plant water stress tolerance by increasing the water use efficiency and the nutrient concentrations of yield.