The aim of this study was to determine the effect of deficit irrigation on yield of onion grown under unheated greenhouse condition. The research was carried out at the Agricultural Research Station of Yenisehir High School of Uludag University in Bursa, Turkey, in 2007. In the study, water was applied to onion as 100, 75, 50, 25 and 0% (as control) of evaporation from a Class A Pan corresponding to 2-day irrigation frequency. Irrigation water applied to crops ranged from 65 to 362 mm, and water consumption ranged from 95 to 372 mm. The effect of irrigation water level on the yield, bulb heigth, bulb diameter, bulb weight and dry matter ratio were found to be significant. The highest yields were 51 and 39 t ha(-1) for the K1(cp) and K2(cp) treatments, respectively. Crop yield response factor (k(y)) was found as 1.060. The highest values for water use efficiency (WUE) and irrigation water use efficiency (IWUE) were found to be 13.84 and 12.71 kg mm(-1) for the K3(cp) treatment. Under the conditions that water resources are scarce, it can be recommended that K3(cp) treatment is most suitable as a water application level for cucumber irrigation by drip irrigation under unheated greenhouse condition.