At the same time of providing a huge amount of energy to the world population (social sustainability) and global economy (economic sustainability), the fuel itself also releases a great amount of emissions to the environment the world people live in in the forms of gaseous pollutants (SOx, NOx, CO, CO2, CH4, etc.) and ash compositions (Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SO3, SiO2, TiO2, etc.), seriously impacting the environment (environmental sustainability) for the world population and global economy. Sustainability generally encompasses economic sustainability, environmental sustainability, and social sustainability, and all of these are significantly related to the energy/resource sustainability. This study addresses the sustainability of fuel from the viewpoint of exergy. It is demonstrated that the energy of a fuel is best evaluated by its chemical exergy, and the environmental impact of a fuel can be assessed through the chemical exergy of its emissions (the specific impacts such as toxicity or greenhouse effect are not detailed). Then, the sustainability of fuel can be understood from the viewpoint of exergy through three ways: (a) high chemical exergy of the fuel, (b) high exergy efficiency of the fuel conversion process, and (c) low chemical exergy of the emissions.