In Middle Awash, cotton growers normally irrigate their crops based on their experience with respect to crop stage, soil condition, appearance of the crop and seasonal fluctuation of volume of Awash River. For good performance of the crop and to use the water coming to the farm to the maximum efficiency possible, there has to be optimal combinations of irrigation water depth and irrigation intervals. Further more, N fertilization of cotton has not been common in this area. Hence this investigation was planned to determine the optimum combinations of amount and frequency of irrigation water and N rate. Three levels of cumulative depth of irrigation water, three levels of irrigation frequencies and four rates of N fertilizer were investigated. The optimal combination for best performance of the crop was observed at shortest irrigation interval (10 days) and higher irrigation water depths, 65 and 75 cm with N rates of 60 - 90 kg N ha-1.