AbstractCompetition of winter and summer annual crops for irrigation water is a vital problem in areas of short water supply. Proper time and frequency of irrigation are essential for the most effective use of water and optimum grain yield of barley (Hordeum vulgare L.). This experiment was conducted in a calciorthids clay loam soil and in a region with 10‐year average rainfall and evaporation of 252 and 1,453 mm, respectively. Complete randomized design was used to compare the effects of no irrigation (I0); one irrigation at either heading (I1), flowering (I2) or dough stage (I3); two irrigations at either heading and flowering (I12), heading and dough stage (I13) or flowering and dough stage (I23); and three irrigations at the heading, flowering, and dough stage (I123) on the grain yield, yield components and water use efficiency of Zarjo barley for 2 years. One irrigation at the flowering stage resulted in an average grain yield increase of 35% compared with no irrigation treatment and was found to be preferable to irrigations at any other stages of growth with respect to both grain yield production and water use efficiency. The lower grain yields at the no irrigation and the one irrigation at dough stage treatments were resulted mainly from the lower weight of 1,000‐seed. Flag leaf xylem water potential lower than −25.8 bars possibly reduced the translation of assimilates into the grain and caused the lower weight of 1,000‐seed. The straw yield was not affected by the irrigation treatments since they were imposed after completion of vegetative growth. One irrigation a the flowering period may be suggested for obtaining both higher water use efficiency and grain yield of barley as winter‐annual crop under irrigation in the southern region of Iran.