This study was conducted to monitor the health conditions of farmed olive flounder according to water temperature. A bio-logger was inserted near the hearts of olive flounder to observe changes in heart rate in response to water temperature under two scenarios. In Experiment 1, the water temperature was raised (1°C/day) from 15 °C to 25°C. In Experiment 2, the water temperature was constant (28°C). Control fish were cultivated at 24°C. All logged heart rate measurements were graded with a data verification quality index (QI) to calibrate the electrocardiogram data. All experiments were done in duplicate. In Experiment 1, the average heart rates at 15 and 25°C were 35± 8.70 beats min-1 and 59 ± 20.99 beats min-1, respectively, a significant (p < 0.05) increase of 69%. No significant changes in heart rate relative to the 15°C baseline were observed from 15°C to 22°C (p > 0.05), whereas a significant difference was observed at 23°C (p < 0.05). In Experiment 2, the heart rate of flounder at 28°C fluctuated (66–69 beats min-1). In control fish, heart rate remained constant, with an average of 46 ± 1.66 beats min-1. Notably, the heart rate was higher in Experiment 2 than in Experiment 1, suggesting that cardiac output increases because the oxygen requirement of olive flounder increases with water temperature conditions, especially during growth at high temperatures.