A heat pipe is mainly composed of a vacuum envelope, a wick structure and a working fluid. The heat pipe is totally evacuated and then filled again with a little quantity of working fluid (coolant), an amount just to fill the wick. Because the coolant is the vital member in the heat pipe, the pressure inside the pipe is the same as the saturation pressure accompanied with the heat pipe temperature. When the heat enters at the evaporator, equilibrium is disturbed, this will cause vapor to generate at a little higher pressure and temperature. The higher pressure leads vapor to travel to the condenser end where the slightly lower temperature leads the vapor to condensate and release its latent heat of vaporization. The condensed fluid after that is pumped towards the evaporator by the capillary forces initiated in the wick structure. This going on cycle can transfer large amounts of heat even with very small thermal gradients. A heat pipe's operation is passive, being leaded only by the heat that it transfers, which consequently will result in high reliability and long life.