Egypt is a semi-arid region and consequently agriculture in Egypt depends on specific sources of water; the extremely important of which is the River Nile. So, the rationalization of water of irrigation is essential and the evaluation of water requirements is necessary for each crop. Water consumption can be calculated from the different methods. The volumetric lysimeter is one of the direct methods to estimate the water requirements. .The estimation of water requirements is dependent on calculating the crop coefficient (ETc = ETo × Kc), where the crop coefficient (Kc) was registered as the proportion of potential and crop ET and the reference evapotranspiration (ETo) was determined according to the Food and Agricultural Organization (FAO) by Penman-Monteith strategy. Therefore, the objectives of this study were to estimate crop coefficient (Kc) for different growth stages of onion (Allium cepa L.). The study was carried out at the Experimental Farm of the Institute of Post-Graduate Studies and Research in Arid Lands, Ain Shams University at Shubra El Kheima, Qalyubia Governorate, Egypt. The experiment was conducted in 2014/2015 and 2015/2016 seasons under conditions of volumetric lysimeter (dimensions 1 × 1 × 1m) and filled with three types of soil (clay, sandy clay and sand) with three levels of water requirement (75,100, and 125% of ETc) for the crop. Onion cv. Giza 61 was used. The results indicated that the increases of irrigation water levels (100 and 125% ETc) were the best treatments for some growth parameters such as plant length, bulb diameter, content of Cu in onion bulb, total and marketable yields in clay soil, but the application of 75% ETc was the best treatment for dry matter of bulb, T.S.S and bulbing ratio in sandy clay soil. The averages of crop coefficient values were 0.47, 0.5, 1.24 and 0.99 during establishment, development, mid-season and end-season growth stages, respectively. Water use efficiency (WUE) for onion was determined for all treatments and the effects showed that the application of 100% ETc gave the highest values.