This study has focused on the systematical investigation of effect of mass fraction and size of multi-walled carbon nanotubes (MWCNTs) doped in phase change material (PCM) on thermal properties such as thermal conductivity, melting/solidification temperatures and latent heats. Thermal conductivity, melting/solidification temperatures and latent heats of MWCNTs/PCM composites with three different diameters and two different lengths, obtained by doping MWCNTs into PCM at the mass fraction of 1-5%, were evaluated according to the criteria such as particle size and mass fraction. The results demonstrated that not only mass fractions but also size of MWCNTs are effective on the thermal properties of the composites. It was concluded that increase in diameter and length of MWCNTs positively affects enhancement of thermal conductivity; on the other hand, it does not cause a significant change at melting/solidification temperatures. In addition to these, a decline was observed at melting/solidification latent heats of MWCNTs/PCM composites, depending on doped mass fractions of MWCNTs.