Transportation sector is the major consumer of oil products. Since consumption of gasoline has increased
during the recent years, pollution of air and warming up of the earth as result of the production of green house
gases, has increased. Natural gas, due to its environment friendliness as one of its characteristics, can become
the dominant fuel in the transportation sector. In this study, we have evaluated the amount of Green House
Gases produced from transportation sector using Clean Development Mechanism formuala. Then external and
social costs from produced Green House Gases, the economies of the owner`s and governmental view and the
present capacity of CNG stations to replace the CNG for other conventional fuels in zanjan city, were studied
according to the variety of sceneries (increasing amount of bi-fule vehicles 25%, 50%, 75%, 90%, respectedly).
The results show that the most amount of Greenhouse Gasses (GHGs) belong to the vehicles with conventional
petrol engines which exceeds 2.08×1011 CO2 tonnage in a year imposing external and social costs which
amount to $2.9×1012 and $1.6 ×1012, respectively, in just one single year. The least amount of (GHGs) belong
to the CNG fueled vehicles with 3.4×109 CO2 tonnage in a year imposing external and social costs that would
be $1×1010 and 1.4 ×1011, respectively, in just one single year. This study implies that the cost is mounting up,
in the owner`s and governmental view. The present capacity of CNG stations to replace the CNG for other
conventional fuels is just suited to the existing conditions in this city and the number of bi-fuel vehicles, but
in other sceneries which requires that the city must be provided with other stations, should be disused in an
independent study.