This paper presents the design and the experimental tests of an ultra-capacitor pack used as an energy storage system in a small hybrid electric vehicle. This design consists in calculating the capacitance and the power of the capacitors, which are necessary to store kinetic energy during deceleration of the vehicle and then to accelerate the vehicle. In a second part, experimental tests are achieved to determine the real parameters of the capacitor pack electric model [1] in order to validate the manufacturer data. Influence of temperature on those experimental data is investigated too. Finally, the authors determine the energetic behaviour of the ultra-capacitors: they have measured an efficiency of around 95% during capacitors charge and discharge.