Recently, It's common to adopt electric power source rather than hydraulic power source for gun/turret driving system since solid state is developed and more and more advantages of electric system are recognized. One of the important components for electric power source is energy storage unit which is used to compensate for peak power and store regenerative power. The ultracapacitor is an electrochemical capacitor having energy density much greater than that of standard electrolytic capacitors. The ultracapacitor is one of the good solutions for energy storage unit. However, durability affected by heating is one of the key issues concerning ultracapacitor applications. For electric gun/turret driving system, ultracapacitors are charged and discharged at high current which causes ultracapacitors heating. This paper describes heat dissipation design of energy storage unit with ultracapacitor and verify a temperature characteristic improvement with conducting a thermal analysis.