Water use efficiency (WUE),which is the ratio of CO_2 assimilation to water loss, has been recognized as an important parameter in water use of plant individuals or ecosystems. As a classic index of carbon-water coupling at various temporal and spatial scales, from leaves to ecosystems, it is useful to determine the adaptation of plants to climate change. The North China Plain is the main maize (Zea mays L.) production region in China,where water is the key resource for optimum crop production. Leaf water use efficiency is defined as the ratio of P_n to T_r,which is mostly affected by biological and environmental factors. Although many findings about leaf water use efficiency have been published previously, limited attention has been focused on the water use efficiency relationship between leaves from the same individual plant under water stress during the whole growth period. Therefore, to understand changes of water use efficiency among different leaf positions at the leaf scale and investigate the main factors controlling leaf water use efficiency under water stress,an experiment with different water treatments (only one time irrigation, water controlled at seven and three leaf stages in 2013 and 2014, respectively) for maize was designed and conducted at the Gucheng Agro-meteorological Field Scientific Experiment Base, China Meteorological Administration (115°40'E,39°08'N) in northern China. The amount of water irrigated in the experiment was in accordance with the total average precipitation during late July and July during 19802010 separately in 2013 and 2014. The results showed that there was an obvious pattern for the first opened leaf's water use efficiency during progressive water stress, as it had two peaks at the jolting and milking stage. Then, leaf water use efficiency in the later growth stage was higher than that in the earlier stage ; moderate water deficit improves water use efficiency, but the quantitative relationship between soil water availability and plant water use efficiency needs further investigation. Moreover, leaf water use efficiency was closely connected to its physiological and ecological characteristics,such as leaf water content and specific leaf area. Furthermore,leaf water use efficiency among different leaf positions along the maize plant showed little variation under nearly identical environments,such as soil water content and atmospheric conditions,which means leaf water use efficiency had spatial stability or conservation regardless of leaf ages to some degree. Meanwhile, the results also showed the coordinative changes between the photosynthetic and transpiration rates had mostly contributed to the leaf water use efficiency stability at different leaf positions. The findings provide information for plant-water relationship research and facilitate a mechanistic understanding of the carbon-water relationships at a leaf scale under water stress conditions,and lay a foundation for and up-scaled study of water use efficiency.