a b s t r a c tThis study presents the effect of Al 2 O 3 nanofluid (NF) on thermal performance of screen mesh heat pipe in cooling applications. Three cylindrical copper heat pipes of 200 mm length and 6.35 mm outer diameter containing two layers of screen mesh were fabricated and tested with distilled water and water based Al 2 O 3 NF with mass concentrations of 5% and 10% as working fluids. To study the effect of NF on the heat pipes thermal performance, the heat input is increased and then decreased consecutively and the heat pipes surface temperatures are measured at steady state conditions. Results show that using 5 wt.% of Al 2 O 3 NF improves the thermal performance of the heat pipe for increasing and decreasing heat fluxes compared with distilled water, while utilizing 10 wt.% of Al 2 O 3 NF deteriorates the heat pipe thermal performance. For heat pipe with 5 wt.% Al 2 O 3 NF the reduction in thermal resistance of the heat pipe is found to be between 6% and 24% for increasing and between 20% and 55% for decreasing heat fluxes, while the thermal resistance increased between 187% and 206% for increasing and between 155% and 175% for decreasing steps in heat pipe with 10 wt.% of Al 2 O 3 NF.