In this study, a paraffin/diatomite/MWCNTs (multi-wall carbon nanotubes) composite PCM (phase change material) was tailor-made for further applications in producing thermal energy storage cement-based composites. For the purpose of selecting an optimum diatomite as the supporting matrix for the new composite PCM, the pozzolanic reactivity and paraffin absorption capacity of various diatomite materials were evaluated. It was shown that the diatomite material calcined at 600. °C for 2. h has the best pozzolanic reactivity and comparable paraffin absorption capacity compared to other diatomite candidates. It, therefore, was used as the supporting matrix of the new paraffin/diatomite/MWCNTs composite PCM. The thermal properties of this fabricated composite PCM were determined by the DSC (differential scanning calorimetry) method. It was revealed that the composite PCM has a melting temperature of 27.12. °C and latent heat of 89.40. J/g. In addition, experimental results from FTIR (Fourier transform infrared) and TGA (thermogravimetric analysis) on this composite PCM showed that it has good chemical compatibility and thermal stability. Moreover, compared to paraffin/diatomite composite PCM, the use of MWCNTs was found to have clear beneficial effects for improving the thermal conductivity and heat storage/release rates, without affecting the thermal properties, chemical compatibility and thermal stability. © 2014 Elsevier Ltd.