RAMS and HYPACT codes were originally developed for the simulation of meso-scale phenomena. Two kinds of new functions were integrated with RAMS and HYPACT codes to simulate air flow and gas diffusion in micro-scale region. First, buildings were approximated with a drag force of the momentum equations. Calculated results showed a good agreement with wind tunnel data of Hamburg University. Next, turbulence model was improved to simulate the surface layer around steep terrain. Calculated results showed a good agreement with our wind tunnel data of isolated hill and with observed meteorological data in complicated terrain. The improved RAMS and HYPACT codes are used for the emergency response system of all nuclear power stations in Japan.