The main function of junction box on a solar photovoltaic module is to transmit the electricity from solar photovoltaic panel to the load. The invented smart junction box is designed for the following purposes: (1) to optimize efficiency through auto control, (2) to reduce the vicious effect of the cloud and dust on electricity output, and (3) to prevent the battery from overcharging. When actuated, the temperature of inner transistor is raised up to 158 °C because it consumes more solar electric power than a conventional junction box. Thus, a more effective thermal diffusion system is invented to cool down inner transistor to avoid failure. It is found that the presented heat plate diffusion system modified from heat pipe is capable of satisfying all required thermal diffusion conditions of the smart junction box.