Sustainability of groundwater resources is a main factor for proper management of water resources in arid areas. Detailed information on groundwater renewability is vital for sustainable development of groundwater resources in arid basins in northwestern China. Radioisotopic dating of groundwater can provide us with significant information about the circulation time and renewability of groundwater. The present study is designed to show how isotopic measurements ( 3H and 14C) can be integrated to calculate groundwater ages in the Heihe valley alluvial basin, and then discuss the renewability of groundwater. The results suggest that the groundwater ages in unconfined aquifers of the basins are less than 50 a, and those in the piedmont plain and the Zhangye basin are less than 40 a. These waters are renewable resources. The groundwater ages in confined aquifers is more than 2000 a, such as 5486~8779 a, in Ejina. These waters could be regarded as poorly renewable resources. However, the groundwater ages in confined aquifers near the Heihe River in the Zhangye basin are less than 50 a, and thus the groundwater is renewable. The above understanding is of significance not only for the management of water resources and ecological environment, but also for the study of sustainable development of ground water resources in similar inland areas.