Abstract: Oxygen toxicity is one of the major risk factors in the development of the chronic lung disease or bronchopulmonary dysplasia in premature infants. Using proteomic analysis, we discovered mitochondrial aldehyde dehydrogenase (mtALDH or ALDH2) was down-regulated in neonatal rat lung after hyperoxic exposure. To study the role of mtALDH in hyperoxic lung injury, we overexpressed mtALDH in human lung epithelial cells (A549) and found that mtALDH significantly reduced hyperoxia-induced cell death. Compared to control cells (Neo-A549), the necrotic cell death in mtALDH overexpressing cells (mtALDH-A549) decreased from 25.3% to 6.5%, 50.5% to 9.1% and 52.4% to 15.06% after 24-, 48- and 72-hour hyperoxic exposure, respectively. The levels of intracellular and mitochondria-derived reactive oxygen species (ROS) in mtALDH-A549 cells after hyperoxic exposure were significantly lowered compared to Neo-A549 cells.