Multi-Scale Simulator for the Geoenvironment (MSSG) is a coupled non-hydrostatic atmosphere-ocean-land global circulation model. MSSG is optimized to be run on the Earth Simulator with high computational performance and it is designed to be available with flexibility for different space and time scales. MSSG can simulate phenomena not only of global circulation but of ultra high small scale such as several meters for horizontal resolution. An atmospheric component of MSSG is a non-hydrostatic global/regional atmosphere circulation model. MSSG-A is compromised of fully compressive flux form. Smagorinsky-Lilly type parameterizations for sub-grid scale mixing, surface fluxes of Zhang and Blackadar type model and cloud microphysics with mixed phases by Reisner are implemented. Cloud-radiation scheme for long wave and shortwave interactions with both explicit cloud and clear-air are adopted. Three dimensional radiation process mentioned above was introduced to MSSG for the purpose of understand heat radiation mechanism in urban canyon area. Three dimensional radiation processes among walls on buildings and roads is considered as strong impacts to heat storage system and being indispensable for simulation/understanding of heat island phenomena. In this research, Monte Carlo method is adopted to calculate the building form coefficient. Although Monte Carlo method is required to high computational cost, the overhead situation in radiation processes is automatically detected through the processes of pursuing individual particles. Furthermore, in calculation of the building form coefficient, the coefficients with high accuracy can be computed by using the fewer numbers of particles using characteristics of its symmetry. The radiation heat transmission process on an arbitrary grid should consider the reflection of the heat radiation on other arbitrary grid. Those all processes are considered in calculation of heat radiation energy. Preliminary simulation (Fig.1) started from an initial state settled at 15:00 on 5th August in 2005 with 5m horizontal resolution. Initial thermal condition was set taking account of rate of shade in a day. Simulations results (Fig.2) show that heat tends to be stored in lower level in urban canyon layers and the tendency was appeared on lower layers for the all direction comparing with simulation results without three dimensional radiation. Those results suggest that radiation system may impact to change of vertical convection and heat storage system in urban canyon area. In addition, efficient computational schemes will be also discussed in this presentation.