The low thermal conductivities of organic phase-change materials must be increased so that they can be used effectively in practice. In this work, it is aimed to enhance the thermal conductivity of PCM by adding nanoparticles into PCM. The effect of used nanoparticle type, size and shape on the thermal properties such as thermal conductivity, latent heat, melting / solidification temperatures has been systematically investigated. The thermal conductivity enhancement of composites doped with carbon based nanoparticle were found to be higher than that of the composites doped with metal oxide nanoparticles. 154.9% improvement was achieved in the thermal conductivity of the phase change material composite doped with 5% GNP. It has been observed that there is no significant change in melting/solidification temperatures (<1.5 degrees C) depending on the doped metal oxide or carbon-based nanoparticles, but deterioration up to 20% in latent heat. However, ZnO nanoparticles have been found to have the effect of enhancing latent heats. In addition to the improvements achieved in the thermal conductivities of phase change materials, their stability are protected over the long term. This result show that it can be used as an energy storage material.