The authors have made a thorough study of what sort of countermeasures can be taken to mitigate the heat island effect by using the evaluation method of the sea breeze effects on urban thermal environment. As the result, the implementation of the countermeasure in the central area of Tokyo has a great effect on reducing exhaust warm air at there and increasing the influence of sea breeze at the inland. These results show the effectiveness of the countermeasures for improving the urban thermal environment of inland in daytime of summer.