The protective effect of Cordyceps militaris against high glucose-induced oxidative stress in human umbilical vein endothelial cells (HUVECs), as compared with Cordyceps sinensis, was examined. The cytotoxicity of HUVECs induced by 40 mM glucose was ameliorated by water extracts of C. militaris (CME) and water extracts of C. sinensis (CSE). CME and CSE inhibited the increase in ROS and NO in HUVECs induced by 40 mM high glucose. Moreover, CME increased the Bcl-2/Bax ratio, modulated the mitochondrial membrane potential and reduced the caspase-3 activity in high glucose-induced HUVECs. In addition, cordycepin, a component of CME and CSE, displayed protective effects against oxidative stress, which was partly responsible for the cytoprotective effects of CME and CSE against high glucose-induced oxidative stress in HUVECs. Overall, the obtained results show C. militaris helps preventing diabetic endothelial dysfunction and related complications.