Abstract: Abstract: Different-root-grafting has been reported to be an effective agricultural practice to improve water use efficiency in China, however, little is known about the effect of different-root-grafting on water use efficiency under moderate deficient irrigation. To study the response of different-root-grafting (G1) to moderate deficient irrigation (12 m3/667m2, W1) and provide data support for agronomic water-saving practices in protected cultivation, this research used cucumber (Cucumis sativas L. No. c.v. zhongnong No.26) and pumpkin (Cucurbita moschata Duch.) as scion and rootstock, respectively. And self-root grafted cucumber (G0, this research used self-root grafted-cucumber as grafted-cucumber control since there is a wound healing process for different-root grafted-cucumber during grafting) under appropriate irrigation (20 m3/667m2, W2) was used as control, and then four different treatments (i.e. W1G0, W1G1, W2G0, W2G1) were conducted to study the responses of different-root grafted-cucumber root growth and distribution, cucumber fruit yield, plant dry matter accumulation, fruit quality and water use efficiency to moderate deficient irrigation. A typical commercial greenhouse in Fangshan county, Beijing was randomly selected for field experiment from April 2011 to January 2012. The greenhouse was covered with polyethylene film without supplementary lighting or heating. Cucumber seedlings with two leaves were transplanted by hand, with double rows of 80-cm row pacing and 33-cm plant spacing. Different irrigation treatments were conducted after the resuming growth of cucumber seedlings after transplantation. The irrigation treatments W1 and W2 were started at the same time and the irrigation time was designed according to the appearance of wilting of self-root grafted cucumber seedlings under the irrigation treatment W2 and also the reviving time needed by cucumber plant. In order to prevent the test error caused by side permeability, we buried 50-cm plastic film at vertical depth between different treatment plots. Once the final harvest was completed, cucumber roots were immediately sampled from the cucumber rhizosphere. The root samples were washed with tap water, and then scanned using the EPSON EXPRESSION 4990 root scanner, and finally analyzed using the WinRHIZO root analysis software. The results showed that under moderate deficient irrigation, different-root-grafting enhanced the root distribution of photosynthetic product, increased fruit quality, and maintained the cucumber yield, however, irrigation quantity was reduced by 64 m3/667m2 in winter-spring season and 40 m3/667m2 in autumn-winter season, and significantly increased water use efficiency ,when compared to control. Root dry matter and water use efficiency under different-root-grafting was 14.2%, 42.1%-53.9% higher than that under the control, respectively. Compared to self-root grafted cucumber, different-root-grafted cucumber significantly improved cucumber root growth, the total root length, total plant biomass and cucumber fruit yield by 17.9%-27.5%, 11.6%-15.2% and 7.5%-29.6% under moderate deficient irrigation condition, respectively. At the same time, the root lengths of 15-45 cm root horizon and water use efficiency were significantly higher under different-root-grafting than under self-root-grafting. This result indicated that different-root-grafting can improve water use efficiency, and also demonstrated that different-root-grafting can be used as one of the major water saving techniques in greenhouse cucumber production, especially in intensive cucumber production systems in China.