Abstract: In the present study we analyze the pollutant dispersion on Paraiba River Valley. This region is placed between two biggest cities in Brazil: São Paulo and Rio de Janeiro. The Valley is also between two mountains ridge: Serra da Mantiqueira (Mantiqueira ridge) and Serra do Mar (Sea ridge). In the Valley, important cities are located with high population (for example São José dos Campos and Taubate cities with more than 800.000 people), with important industrial area. There are many pollutant sources in this region: industries and a high-way with the heaviest traffic in the country. Our interest here is not to simulate one specific event, but to have a scenario of the pollutant diffusion from typical meteorological situations. The intention is to simulate an average condition to understand the evolution of the pollutant dispersion under different weather conditions, over a complex terrain. The mesoscale meteorological model BRAMS is employed to simulate the wind field, and other parameters such as boundary layer height, wind velocity scale, and so on. The pollutant dispersion is simulate by using the CALPUFF – EPA/USA model. The goal here is to use a simple dispersion model, with low computational cost. The motivation of this research is to help the cities administration to have good information on the impact of pollutant on the population looking at a better urban planning.