This paper deals with the feasibility of hybrid transit city bus combining zero emissions with efficiency of power system. The vehicle system configuration uses four in-wheel electric traction motors powered by an hybrid unit composed by functional linking of hydrogen fuel cell and flywheel energy storage system. The proposed system design is attempted to minimize the fuel cell power on board of vehicle satisfying the transient loads by the flywheel energy storage device. Finally a small sized city bus of 20 passengers of carrying capacity is considered and the main components of power system are defined and dimensioned within European urban standard cycle.