This paper aims at assessing the performance of a waste heat-to-power plant by means of an ORC (Organic Rankine Cycle) system coupled with two-phase closed thermosyphons (or gravity-assisted heat pipes or wickless heat pipes). The heat exchanger, made up of two-phase closed thermosyphons, is used for conveying heat from exhaust stream to ORC working fluid. In reality, a hot oil loop or a pressurized hot water loop or a saturated steam loop or even a direct evaporator is often used to transfer heat from the heat source to the ORC system. However, installing a heat exchanger directly in the hot gas paths of ORC system evokes the concerns for the flammability and/or toxicity of organic working fluids especially when the heat source has a relatively high temperature. Also the use of an intermediate heat carrier loop such as thermal oil or saturated steam or pressurized water loop is costly and involves installation of comparatively heavy components. In principle, the use of two-phase closed thermosyphons for capturing and transporting heat from a waste heat source to organic working fluid is less expensive than utilizing an intermediate heat transfer loop and eliminates safety issues as in the case of direct installation of an ORC evaporator in the hot gas paths. Int. J. of Thermodynamics (IJoT) Vol. 20 (No. 2) / 83 function of figure of merit (FOMt), which has the dimension 3/4 5/2 kg Ks , defined as follow [3]: 1/4 23 Q , co  Int. J. of Thermodynamics (IJoT) Vol. 20 (No. 2) / 85