The maximum electrical efficiency of fuel cell system, η max e , is important for the understanding and development of the fuel cell technology. Attempt is made to build a theory for η max e by considering the energy requirement of heating the fuel and air streams to the fuel cell operating temperature T. A general thermodynamic analysis is performed and the energy balances for the overall operating processes of a fuel cell system are established. Explicit expressions for the determination of η max e are deduced. Unlike the Carnot efficiency, η max e is found to be fuel specific. Except for hydrogen fuel, chemical equilibrium calculations are necessary to compute η max e. Analytical solutions for the chemical equilibrium of alkane fuels are presented. The theoretical model is used to analyze the effects of T and the steam contents of CH 4 , C 3 H 8 , and H 2 on η max e for systems with various degrees of waste heat recovery. Contrary to the common perception concerning methane and propane fuels, η max e decreases substantially with the increase of T. Moreover, η max e of hydrogen fuel can be higher than that of methane and propane fuels for a system with a medium level of waste heat recovery and operated at 700 • C≤T ≤900 • C.