We performed linear programming for optimization of fossil fuel supply in 2000 in Turkey. For this, an exergy analysis is made because the second law of thermodynamics takes into account the quality of energy as well as quantity of energy. Our analyses showed that the interfuel substitution between different fossil fuels will lead to a best energy mix of the country. The total retail price of fossil fuels can be lowered to 11.349 billion US$ from 13.012 billion US$ by increasing the domestic production of oil, lignite, and hard coal and by decreasing imports. The remaining demand can be met by natural gas imports. In conclusion, our analysis showed that a reduction of 1.663 billion US$ in fossil fuel cost can be made possible by giving more emphasis on domestic production, particularly of oil, lignite and hard coal.