The stratification of upper layer of coastal water can be regarded as an unused energy source in summer. Moreover, the coastal water has capability to store the urban heat owing to the heat capacity difference to the atmosphere. The numerical simulation of the urban heat release into coastal zone is performed. The estimated urban heat along Osaka Bay is 1kGJ/h within the distance from 500 m to 5000 m from the coastal line. The urban heat released with 2000 m from the coastal line had little influence on the sea surface temperature of Osaka Bay.