Abstract Objective Oxidative stress-induced cell damage is considered to be an important factor in aging and neurodegenerative diseases such as Alzheimer's disease (AD), Age-related macular degeneration (AMD). Metformin, a most commonly used anti-type 2 diabetes drug, has recently been shown to extend the lifespan of various species and is therefore regarded as a potential anti-aging drug. However, the antioxidant effects of metformin and its underlying mechanisms in neurodegenerative diseases are unclear. Methods The protective effect of metformin against H2O2-induced cell viability loss was tested by using MTT assay. Flow cytometry, caspase3 activation assay, lactate dehydrogenase (LDH release) assay, Hoechst staining were used to evaluate the protective effect of metformin against H2O2-induced apoptosis. Intracellular reactive oxygen species (ROS) intracellular levels was assessed by Cell ROXs Deep Red Reagent. Changes in mitochondrial membrane potential were measured by JC-1 staining. The possible signaling pathways involved in the neuroprotective effect of metformin were checked by western blot analysis. Results We found that metformin protects neuronal cells (PC12/D407) from H2O2-induced cell death at clinically relevant concentrations. Metformin treatment significantly reversed changes in intracellular ROS, lactate dehydrogenase and mitochondrial membrane potential caused by H2O2. Hoechst staining analysis and flow cytometry analysis showed that metformin significantly reduced apoptosis of neuronal cells exposed to H2O2. Western blot analysis further demonstrated that metformin stimulated the phosphorylation and activation of AMPK in neuronal cells, and the protective effects of metformin were blocked by knockdown AMPKα or inhibiting AMPK using compound C. Similar results were obtained in primary cultured hippocampal neurons and primary retinal pigmented epithelium (RPE) cells. Conclusions Taken together, these results indicate that metformin is capable of at least partially protecting neuronal cells from oxidative damage by activation of AMPK. Because metformin is relatively inexpensive and has fewer clinical side effects, our findings support its potential as a drug for the prevention and treatment of aging and aging-related diseases.