Abstract: Based on the fundamental definition of the electrical efficiency and the thermodynamics principle, a theoretical expression for the maximum electricity efficiency of a fuel cell system was derived. The characteristics of cell voltage operating at the maximum electrical efficiency and the energy required for heating the fuel and air were considered in deriving the maximum electricity efficiency expression. The values of the maximum electricity efficiency for hydrogen fuel at different operating temperatures were presented. The relationships between the maximum electricity efficiency and the working temperature, fuel utilization and the fuel steam content were analyzed.