Baia da Ilha Grande is a bay located south Rio de Janeiro city, Brazil. It is a very important water body in regard to economic and ecological purposes, since it has a major commercial harbor, an oil terminal, many industries and two nuclear power plants. It is also one of the most important tourist resorts of Rio. An applied research study was made by the Marine and Atmospheric Processes Modeling Laboratory (LAMMA) of IJniversidade Federal do Rio de Janeiro to the Brazilian state oil company Petrobras in order to evaluate the hydrodynamic circulation in the bay and fate of oil spills that could take place in the Petrobras oil terminal, It was used the 3-D barc~clinicoceanic numerical model Princeton Ocean Model (POM) to simulate the current field using both tidal and wind forcing, The oil spill model Nicoil developed by LAMMA that considers the most important weathering processes was implemented in this region. Methods and results are presented,