To assessment of water use efficiency in related to yield and yield components of corn in deficit irrigation condition, this field experimental was conducted in 2008 and 2009 cropping seasons in the under semi-warm climate condition in south west of Iran. The treatments were consisted three irrigation levels (based on 100, 80, 60 percent of full irrigation) and water stress in five levels (full irrigation and irrigation-off at the 8-leaf, the 12 leaf, the tasselling and grain milky stages) applied in factorial experiment, using randomize complete block design with three replications. Results showed that the effect of deficit irrigation and water stress were significant on grain yield. Both grain and biological yields reduced in response to stress intensity.. The highest grain and biological yields were obtained in full irrigation. The combination treatment of 60 percent of full irrigation with irrigation-off at the tasselling stage reduced the grain yield by 93% compared to the optimum irrigation condition. The highest rates of harvest index (HI) and water use efficiency (WUE) were obtained in full irrigation treatment. According to the results of this study to achieve simultaneously higher water use efficiency and lower grain yield reduction, irrigation based on 80% of full demand of the plant is recommended
