The effects of sudden changes of water temperature (WT) on the stress response and physiological change of the cultured olive flounder in large (FL) and small (FS) size, and fat cod (FC) were examined by manipulating WT (2 types) in a flow through seawater culture system with 6 tanks (water vol. 270 L/tank). The WT was decreased from $20^{circ}C$ to $10^{circ}C$ within 5 hours ($2^{circ}C/hour$) and maintained at $10^{circ}C$ for 21 hours (Exp. I), and it was raised from $20^{circ}C$ to $30^{circ}C$ within 5 hours and maintained at $30^{circ}C$ for 21 hours (Exp.II). In Exp. I, the levels of blood hematocrit at 5 hours ($10^{circ}C$) in FS was significantly decreased from $13.5pm2.0\%;to;11.3pm2.3\%$, but FC at 2.5 hours ($15^{circ}C$) ($19.0pm0.3\%;to;23.2pm3.8\%$) was increased, The blood hemoglobin concentration of all fish in Exp, II was significantly increased until 8 hours after raising WT from $20^{circ}C$ to $30^{circ}C$. In Exp. I and II , the levels of plasma cortisol in FL, FS and FC was changed from $5.2pm8.5ng/mL,;4.4pm4.5ng/mL;and;2.7pm0.4ng/mL$, respectively, before sudden drop and rise of WT. The levels of plasma cortisol of in FL ($164.0pm53.1ng/mL$) and FC ($207.9pm25.4ng/mL$) were significantly increased by the lowering WT sharply during whole experiment. The FL ($12.6pm2.0ng/mL$) and FS ($4.0pm3.9ng/mL$) showed no significant differences in cortisol level according to sudden rise of WT (5 hours). But it in FC ($44.7pm18.2ng/mL$) was increased. In Exp. I, the plasma glucose levels of all fish groups were decreased after 5 hours ($10^{circ}C$), The plasma lactic acid concentration of FL and FS showed no significant differences until 5 hours after raising WT from $20^{circ}C$ to $30^{circ}C$, But it in FC was significantly increased with WT raise.