In cool Mediterranean areas, early sowings which lead to early growth and maturity and escape from terminal heat and drought usually give higher seed yield than late sowings. However, in many years, rains may come late. The objectives of this study were to test whether early sowing followed with an early 30 mm irrigation to ensure earlier emergence will increase barley seed yield, and whether this early irrigation (EI) will lead to higher yield than a late irrigation (LI) with similar amount of water applied after heading. Two experiments (one in 2004-05 and one in 2005-06) each with three replicates were conducted at the Agricultural Research and Educational Centre (33°56’ N, 36°05’ E, 995 m a.s.l.) in the semi-arid northern Bekaa Valley of Lebanon. The long-term annual precipitation of the site is 513 mm, 58% of which falls in December, January and February. In the first experiment, irrigation led to 12 days earlier heading and 45% increase in barley seed yield. In the second experiment, there were 4 treatments: 0EI+0LI, 0EI+LI, EI+0LI, and EI+LI. Rain came just 2 days after the early irrigation, causing little difference in the timing of seedling emergence. However, +EI led to higher leaf area index after heading and shoot dry matter at maturity. As a result, +EI increased seed yield by 33% despite heading only 2 days earlier than 0EI. Late irrigation had no significant effect on seed yield, but did increase numbers of tillers and heads. Water use efficiency of the EI was 1.4 times higher than the LI, and +EL+LI gave the highest yields. Results of the study indicate that in the Bekaa Valley a small amount of irrigation early in the season may contribute to large yield increase and/or save larger amount of irrigation water later in the season.