The purpose of this study was to evaluate the antioxidant effect of rosmarinic acid on the cultured human skin melanoma cells injured by reactive oxygen species. The cytotoxicity and antioxidant effect were analyzed by XTT assay for cell adhesion activity, DPPH-radical scavenging activity, lactate dehydrogenase (LDH) activity after treated of cells with or without hydrogen peroxide ($H_2O_2$) for 1-10 h. $H_2O_2$ decreased the cell adhesion activity in cultured human melanoma cells in a dose- and time-dependent manner after cells were treated with 20-110 ${mu}M$ of $H_2O_2$ for 5-7 h. $IC_{50}$ values of $H_2O_2$ by XTT assay were 90 ${mu}M$, respectively. $H_2O_2$ increased LDH activity dose dependently. $LDH_{50}$ was determined at 60 ${mu}M$ $H_2O_2$ after treatment of cells with 50-90 ${mu}M$ $H_2O_2$ for 8 h. Rosmarinic acid increases cell adhesion activity, DPPH-radical scavenging and decreases LDH activity. The treatment of the cells with $H_2O_2$ shows highly toxic effects on cultured human skin melanoma cells. Rosmarinic acid increases cell adhesion activity, DPPH-radical scavenging activity and decreases LDH activity in the cells treated with $H_2O_2$. It is considered that rosmarinic acid shows antioxidant effect on ROS such as $H_2O_2$.