ObjectiveTo investigate the influence of Fe3O4 magnetic nanoparticles on human keratinocyte cells (HaCaT cells). MethodsHaCaT cells were incubated with different concentrations of Fe3O4 magnetic nanoparticles for 4 h at 37℃ with 5% CO2. Then transmission electron microscopy (TEM) was used to observe the way nanoparticles entering HaCaT cells and the ultrastructure of HaCaT cells. ResultsThe mean diameter of Fe3O4 magnetic nanoparticles was 12 nm. The nanoparticles of different concentrations could enter the HaCaT cells by phagocytosis. After entering the cells the particles were released from phagocytic vesicles and exerted influence on the nearby mitochondria, leading to mitochondria swelling and cristae dissolving. ConclusionFe3O4 magnetic nanoparticles can damage the ultrastructure of mitochondria near the particles in HaCaT cells, and the effect is in a concentration-dependent manner.