Background: Cobalt oxide nanoparticles (Co3 O4 NPs) are increasingly recognized for their utility in biological applications, magnetic resonance imaging, and drug delivery. However, little is known about the toxicity of Co3 O4 NPs in human cells. Methods: We investigated the possible mechanisms of genotoxicity induced by Co3 O4 NPs in human hepatocarcinoma (HepG2) cells. Cell viability, reactive oxygen species (ROS), glutathione, thiobarbituric acid reactive substance, apoptosis, and DNA damage were assessed in HepG2 cells after Co3 O4 NPs and Co2+ exposure.