The wide application of zinc oxide (ZnO) nanoparticles inevitably facilities their release into the terrestrial and agricultural ecosystems, severely affecting crop production. The effects of nano-ZnO (100 mg L-1) on stomatal morphology, gas exchange and key carbon metabolism enzyme activities were investigated in drought-stressed and well-watered maize. Drought stress limited the plant growth, caused stomatal closure, decreased the net photosynthetic rate, while increased the osmolyte concentrations. In drought-stressed maize, the application of nano-ZnO (100 mg L-1) alleviated photosynthetic pigment degradation and benefited the stomatal movement, maintained a higher net photosynthetic rate, and enhanced water use efficiency, promoting the drought tolerance in maize. In addition, the nano-ZnO increased the activities of UDP-glucose pyrophosphorylase, phosphoglucoisomerase and cytoplasmic invertase by 17.8%, 391.5% and 126%, respectively, which enhanced the starch and sucrose biosynthesis and glycolysis metabolism in leaves under drought stress. Manipulation of primary carbohydrate metabolism through nano-ZnO induced regulation of the key enzymes activity benefited the drought tolerance in maize. The nano-ZnO application (100 mg L-1) alleviates the negative effect of drought stress via improving photosynthetic carbon assimilation of maize.