Monitoring and mapping sea water temperature is the very basis of the protection and management of marine ecosystem. Characteristics of sea surface temperature (SST) difference around Gwang-Yang major steel Mill was monitored using 26 Landsat 5 Thematic Mapper and Landsat-7 ETM+ thermal infrared band data from 2000 to 2010. To analyze accuracy of SST from the Landsat-7 ETM+ thermal infrared image, satellite-induced SST was verified by temperature measurement at Yeo-Su tide station. As a result, The mean SST around Gwang-Yang steel mill was 16.8°C in spring, 22.67°C in summer, 15.78°C in autumn and 7.456°C in winter. Based on these results, it was assured that large quantity of warm water has kept being discharged from the Gwang-Yang steel mill.