Hypoxia/reoxygenation insult can be found in many tissues, including heart, brain, and tumor. It is believed that cell death may be resulted after cells were subjected to chronic hypoxia or reoxygenation after chronic hypoxia. The molecular mechanism for reoxygenation induced cell death is so far not clear and will require further study, in particular, to be distinguished from the pathways associated only with chronic hypoxia. In this study, the cell death mechanism in human squamous carcinoma A431 cells after hypoxia/reoxygenation insult is examined. It is demonstrated that although caspase-9 and -3 were activated during both hypoxia and reoxygenation, only those caspases activated during reoxygenation were responsible for reoxygenation induced apoptosis. Activation of caspase-9 and -3 during reoxygenation is believed to be triggered by the ROS formation at the time of reoxygenation. Addition of catalase during reoxygenation was found to attenuate reoxygenation induced apoptosis and caspase activation.