Nanomaterials　have　attracted　considerable　interest　owing　to　their　unique　physicochemical　properties.　The　wide　application　of　nanomaterials　has　raised　many　concerns　about　their　potential　risks　to　human　health　and　the　environment.　Metal　oxide　nanoparticles　(MONPs),　one　of　the　main　members　of　nanomaterials,　have　been　applied　in　various　fields,　such　as　food,　medicine,　cosmetics,　and　sensors.　This　review　highlights　the　bio-toxic　effects　of　widely　applied　MONPs　and　their　underlying　mechanisms.　Two　main　underlying　toxicity　mechanisms,　reactive　oxygen　species　(ROS)-　and　non-ROS-mediated　toxicities,　of　MONPs　have　been　widely　accepted.　ROS　activates　oxidative　stress,　which　leads　to　lipid　peroxidation　and　cell　membrane　damage.　In　addition,　ROS　can　trigger　the　apoptotic　pathway　by　activating　caspase-9　and　-3.　Non-ROS-mediated　toxicity　mechanism　includes　the　effect　of　released　ions,　excessive　accumulation　of　NPs　on　the　cell　surface,　and　combination　of　NPs　with　specific　death　receptors.　Furthermore,　the　combined　toxicity　evaluation　of　some　MONPs　is　also　discussed.　Toxicity　may　dramatically　change　when　nanomaterials　are　used　in　a　combined　system　because　the　characteristics　of　NPs　that　play　a　key　role　in　their　toxicity　such　as　size,　surface　properties,　and　chemical　nature　in　the　complex　system　are　different　from　the　pristine　NPs.