This study investigates how MgO/water nano-fluid affect the thermal performance of a two-phase closed thermo-syphon heat pipe at various states of operation. The present study experimentally demonstrated the effect of using a nano-fluid obtained from MgO on improving the performance of a heat pipe. Triton X-100 nonionic surfactant was used in the study to produce the 5% (vol.) MgO/water nano-fluid via direct-synthesis. A straight copper tube with an inner diameter of 13 mm, the wall thickness of 2 mm and the length of 1 m was used as the heat pipe in the experimental set-up. The nano-fluid was filled up 33.3% (44.5 ml) of the volume of the heat pipe. Three different heating power levels (200W, 300 W and 400 W) were used in the experiments with three different flow rates of cooling water (5, 7.5 and 10 g/s) used in the condenser for cooling the system. An improvement level of 26% at 200W heating power and 7.5 g/s flow rate was reported for the effectiveness of the heat pipe used in the study when the nano-fluid was charged into the system instead of water. The improvement ratio varied with respect to varying heat loads and varying flow rate of condenser cooling water. (C) 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.