In the present study the effect of different size MWCNTs, on the thermal properties such as thermal conductivity, melting/solidification temperatures and latent heats were investigated systematically. Two sizes of MWCNTs that have same diameter but two different lengths as long MWCNTs and short MWCNTs were used at various mass fractions including 1%, 3% and 5% to fabricate MWCNTs/PCM composites. The thermal conductivity, melting/solidification temperatures and latent heats of MWCNTs based composites were evaluated incorporating both particle size and mass concentrations. The experimental results demonstrated that both size and the loading content of the MWCNTs have direct effect on the thermal properties of the samples. The thermal conductivity enhancement of the short MWCNTs was realized to be lower than that of the long MWCNTs. On the other hand, latent heat was measured slightly higher for short MWCNTs than the long MWCNTs.