This paper presents an experimental investigation regarding the use of solid nanoparticles added to water as a working fluid. Tests were made on a thermosyphon heat pipe. The experiment was performed in order to measure the temperature distribution and compare the heat transfer rate of the thermosyphon heat pipe with nanofluid and with DI-water. The iron oxide nanoparticles were obtained by the laser pyrolysis technique. The tested concentration level of nanoparticles is 0%, 2%, and 5.3%. Results show that the addition of 5.3% (by volume) of iron oxide nanoparticles in water presented improved thermal performance compared with the operation with DI-water. (C) 2010 Elsevier Ltd. All rights reserved.