We determined oxidative stress caused by thermal stress in olive flounder Paralichthys olivaceus based on the altered-mRNA expression and enzymatic activity of two key antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), along with monitoring of several other biomarkers. When the fish were exposed to acute thermal change (from $20^{circ}C$ to $25^{circ}C$ and $30^{circ}C$), the expression and activity of both enzymes were significantly higher at elevated temperatures ($25^{circ}C$ and $30^{circ}C$) than at $20^{circ}C$. Lipid peroxidation (LPO) was also higher at $25^{circ}C$ and $30^{circ}C$ than at $20^{circ}C$. In addition, the plasma $H_2O_2$ concentration was significantly increased by thermal stress. Furthermore, we investigated changes due to thermal stress by measuring levels of plasma alanine aminotransferase (AlaAT) and aspartate aminotrasferase (AspAT). Both were significantly increased by thermal stress. As an immune indicator, the lysozyme concentration was lower at $30^{circ}C$ than at $20^{circ}C$, indicating that thermal stress decreases immune function. Therefore, thermal stress could induce oxidative stress and suppress immune function and can cause physiological stress.