In this paper, we present preliminary results of an experimental investigation of melting of nanoparticle-enhanced phase change materials (NePCMs) in a bottom-heated vertical cylindrical cavity. Multi-walled carbon nanotubes (CNTs) were dispersed in 1-dodeconal to prepare NePCM samples with various loadings (0 wt.%, 1 wt.%, and 2 wt.%). Thermal conductivity and viscosity of the NePCM samples were measured at their liquid phase. It was shown that melting is decelerated in the presence of the CNTs as a result of the dramatically increased viscosity, leading to significant degradation of natural convection during melting. This overweighs the enhanced heat conduction afforded by the increased thermal conductivity. Such competing effect between the enhanced heat conduction and weakened natural convection determines the melting rate of NePCMs. (C) 2013 Elsevier Ltd. All rights reserved.