This paper focuses on parametric optimization of a fuel cell/battery range extender implemented in trolleybus. In order to optimize the fuel cell/battery parameters a simulation model of the vehicle dynamics and the powertrain is developed in this study. It allows estimation of driving energy consumption as well as an analysis of energy management between the fuel cell and battery. A modified Millbrook London transport bus (MLTB) driving cycle is used in order to evaluate the driving energy consumption and average power demand. The weight of the trolleybus is varied within the range of 13 700kg to 22 500kg. Battery nominal energy is determined on the bases of driving energy needed per cycle at gross vehicle mass and minimum battery SoC of 0.6 at the end of cycle. Thus, the optimal battery energy accounts to 110 kWh. Finally, the impact of fuel cell size on hydrogen fuel consumption is evaluated. The results reveal that increasing the fuel cell nominal power leads to lower hydrogen consumption. The minimum fuel consumption was observed with the fuel cell of 135 kW when demanded fuel cell power accounts to 37.8 kW. The range extender further will be implemented in a trolleybus to demonstrate the advantage of technology.