Abstract: The 热环present study explains the application of a numerical simulation to investigate the thermal environment of a new riverside residential development in summer. The case study area consists of more than one hundred two-story detached houses built next to a river near Tokyo, Japan. The microclimate factors of residential district have been estimated using the CFD (Computational fluid dynamics) simulation of radiation, conduction and convection. This method leads to an improvement of outdoor thermal environment by manipulating the site design and layout planning scenarios. In total, five scenarios have been simulated for the proposed sustainable development as explained in the paper. Above simulations provided an insight into the mitigation effects of each countermeasure. Key Word: Outdoor Thermal Environment, Residential District Design, Numerical Simulation