With the discovery of oil and gas in the pre-salt Santos and Campos basin, the supply of natural gas (NG) is expected to increase considerably, so the use of compressed natural gas (CNG) in city buses will be an important option for reducing the overall consumption of fossil diesel fuel and a reduction in operating costs in São Paulo and Rio de Janeiro Metropolitan Areas in Brazil.A vehicle with an engine that can run on pure diesel or diesel and CNG has advantage over a vehicle that works exclusively with CNG, because when there is no availability or the lack of CNG, the vehicle / engine operates with diesel only. Another benefit of this technology is the resale value in Brazil, because after the life cycle of use in theses two big cities, Urban Buses are sold country side to small cities where CNG is not available. Another great advantage of this system is its robustness and non-dependence of spark ignition and coils, because the combustion will begin with pilot injection of Diesel.The use of CNG with 90% of substitution ratio will provide a reduction of up to 19% in CO2 emissions (GHG - Greenhouse gas). The Dual fuel engine has the same performance and load response than Diesel engine, with slightly lower efficiency, emitted lower CO2, PM and higher CH4. CH4 is the concern, but the balance of CO2 equivalent (GHG) is better 47,5 g/kWh than Diesel only version. If the availability of Natural Gas and Biomethane increase and the price of CNG and Diesel keep in today baseline in one year, one bus will save R$ 7.978,00 or U$ 4.000,00 and the interest in dual fuel engine will grow.