Nanofluids, which are suspensions of nanoparticles in base fluids, have been introduced as new enhanced media for heat transfer, especially during the last decade. An experimental investigation has been carried out on nanofluid effects on the thermal performance of a medium-sized cylindrical meshed heat pipe, in both transient and steady states. Thermal resistance and response time of the heat pipe are the characteristics of transient and steady states, respectively. In the present study, the response time is defined based on the variation of the heat pipe surface temperature. Suspensions of silver nanoparticles in DI-water were utilized in various concentrations of 50,200 and 600 ppm. The experiments have been performed under heat rates in the range 300-500W. The least utilized concentration of nanofluid resulted in the best performance. Some discussions are presented for reverse and negative effects of high concentration nanofluids. Whereas the heat pipe operation with DI-water was more stable, by using nanofluids, the thermal resistance and response time of the heat pipe decreased up to 30% and 20%, respectively, compared to DI-water. (C) 2011 Elsevier Ltd. All rights reserved.