Water scarcity and competition in the Vietnamese Mekong Delta becomes a great matter of concern under the contexts of climate change and tributary dam construction in upstream countries. Moreover, water productivity or water use efficiency in rice production was quite low in the region due to free-rider problem. Therefore, the study was aimed at measuring water use efficiency at farm level and investigating the factors affecting water use efficiency. The study applied stochastic frontier analysis to estimate water use efficiency for 159 rice farmers in the upstream of the Vietnamese Mekong Delta. The study shows that the average water use efficiency was 18.81%, indicating that rice farmers did consume water very inefficiently. On the average, the rice farmers could reduce their water consumption about 81.19%, keeping their output level and other inputs constant. To improve water use efficiency, the study found that land accumulation or large-scaled production is essential.