A BSTRACT Phase change materials (PCM) are extensively employed as thermal energy storage media in solar thermal energy storage, efficient buildings, and thermal protection of electronics. The most important problem in using PCMs (especially organic PCMs) is their low thermal conductivity that drastically affects the rate of phase change process (melting and solidification of PCM). In the recent years by the development in nanotechnology, high thermal conductivity nanostructured materials have been produced. In the present study, thermal conductivity and rheological behavior of n-octadecane with dispersed nanostructured materials including mesoporous silica (MPSiO 2 ), titanium(IV) oxide (TiO 2 ), carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) at various particle mass fractions and temperatures are experimentally investigated. A maximum thermal conductivity enhancement of almost 5% for TiO 2 and MPSiO 2 were experimentally obtained. MWCNTs and CNFs enhance the thermal conductivity of PCM up to 40% and 20%, respectively. The experimental results on the viscosity showed that for mass fractions of MPSiO 2 and TiO 2 nanoparticles greater than 3% in liquid PCM, the behavior of liquid is non-Newtonian. Also, the viscosity of PCM containing nanoparticles was measured to be increased compared to the liquid PCM. An enclosure with a constant heat flux wall was used to evaluate the melting process. The solid-liquid interfaces were captured and the instantaneous liquid fraction was presented. The effect of natural convection on melting process was studied in terms of the molten fraction and the shape of the solid–liquid interface. The correlations of molten fraction and Nusselt number were obtained and showed that the melting rate would decrease by dispersing TiO 2 nanoparticles in n-octadecan. Solidification experiment was performed using the same enclosure with constant wall temperatures. It was obtained that conduction is the dominant heat transfer mechanism in solidification. The results interestingly represented the improvement in solidification by adding nanoparticles. K EYWORDS Phase change materials, nanostructured materials, thermal conductivity, viscosity, melting heat transfer, solidification.