The Loop Heat Pipes (LHPs) are robust, self-starting and a passive two-phase thermal control system that uses the latent heat of vaporization of an internal working fluid to transfer heat from an evaporator (the heat source) to a condenser (the heat sink). The circulation of the working fluid is accomplished by capillary pressure gradients in a fine porous wick with very small pores. LHPs are rapidly gaining acceptance in the aerospace community and several terrestrial applications are emerging as well. In the present study, a miniature LHP is investigated the thermal performance for spacecraft thermal control system. Tests will be conducted including start-up, low power, power ramp up, high power, rapid power change, and rapid sink temperature change. Finally, we want to demonstrate the potential of LHP to become the next-generation heat transfer device to cool terrestrial devices such as advanced electronic which have high power dissipations. First of all, this paper presents the influence of the gravitational forces on the LHP performance. The present tests performed under steady state condition with three different orientations (horizontal, gravity-assisted, anti-gravity).