This study investigates how fly-ash and alumina nanofluids affect the thermal performance of a two-phase closed thermo-syphon heat pipe at various states of operation. This study experimentally investigates the effect of suspending the fly ash obtained from the flue gas released from the cyclones of the Yatagan thermal power plant (Turkey), which contains various metal oxides such as SiO2, TiO2, Al2O3, Fe2O3, CaO and MgO in varying ratios, in water on improving the thermal performance of a heat pipe. Triton X-100 dispersant was used in the study to produce the 2% (wt) fly-ash/water nanofluid via direct-synthesis. A straight copper tube with an inner diameter of 13 mm, outer diameter of 15 mm and length of 1 m was used as the heat pipe. The nanofluid was filled up with 33.3% (44.2 mL) of the volume of the heat pipe. To be able to make experimental comparisons, three different working fluids prepared under the same conditions in the same heat pipe were tested at three different heating powers (200 W, 300 W and 400 W) and three different coolant water flow rates (5 g/s, 7.5 g/s and 10 g/s). A decrease of 30.1% was achieved in thermal resistance (R) when fly-ash containing nanofluid was used to replace water at a heating power of 400 W and with a coolant water flow rate of 5 g/s; similarly, when alumina nanofluid was used as working fluid, the decrease of 5.2% was obtained in thermal resistance under a heating power of 400 W and coolant water flow rate of 5 g/s. (C) 2016 Elsevier Ltd. All rights reserved.