In this study, the thermal performance of a two-phase closed thermosyphon operating with multi-walled carbon nanotube/deionized water nanofluids is studied. The experiments were conducted in a copper two-phase closed thermosyphon with a length of 450 mm and diameter of 20 mm. A multi-walled carbon nanotube is used with a diameter of 1020 nm and length of 515 microns. The results of experimental tests indicated that with the increasing weight concentration of nanofluids, thermal efficiency improves and Nusselt number and thermal resistance of thermosyphon decrease. In addition, the increment in input power leads to an increase in thermal efficiency and Nusselt number while it reduces thermal resistance.