The performance of conventional thermal systems which use heat transfer fluid (HTF) may be improved by adding nanoparticles within the HTF (nanofluid). However, the heat transfer capabilities of a nanofluid-charged heat pipe are relatively unquantified, and have remained somewhat controversial. This work reviews past studies focusing on the application of nanofluids in various types of heat pipes. Furthermore, full numerical simulation is then presented to account for the effect of nanofluids in a conventional heat pipe. Validation of the numerical model is performed through comparison to experimental tests for various nanoparticles. A revised Merit number is introduced for the performance evaluation of nanofluid-charged heat pipes. A parametric study is also included for the addition of Al2O3, CuO and TiO2 nanoparticles, and provides guidance towards better understanding of nanofluids and their effect on the thermal performance of heat pipes. The optimal nanoparticle concentration of Al2O3, TiO2 and CuO corresponding to the capillary limit for a conventional nanofluid-filled heat pipe was determined to be 25% by vol. for both Al2O3 and TiO2, and 35% for CuO. Overall, a maximum decrease in total thermal resistance was observed to be 83%, 79% and 76% for Al2O3, CuO and TiO2, respectively. (C) 2016 Elsevier Ltd. All rights reserved.