Zinc oxide (ZnO) nanoparticles (NPs) are used in the cosmetic industry in cosmetics and sunscreen. ZnO NPs have been reported to elicit various adverse cellular effects, including cytotoxicity. However, the underlying mechanisms of these adverse effects have not been fully characterized. To investigate the potential of cytotoxicity induced by ZnO NPs, we evaluated cytosolic reactive oxygen species levels in human keratinocyte HaCaT cells treated with ZnO NPs having different surface charges and particle sizes. A short period of treatment (30 min) with 100 nm ZnO NPs resulted in a greater increase of cytosolic ROS levels, compared to treatment with 20 nm ZnO NPs at the same concentration. During a long period of treatment (24 h) with ZnO NPs, intracellular ROS was increased in cells treated with $20{mu}g$/mL 20 nm (+/-) charged ZnO. No significant difference according to differences in surface charge was observed. In addition, total levels of caspase-7 and PARP were decreased by ZnO NPs. These results demonstrated that ZnO NPs could induce ROS mediated apoptosis.