MgO has been used as a popular ceramic skeleton material (CSM) for shape-stablising inorganic salt based composite phase change materials (CPCMs) for medium to high temperature thermal energy storage applications. This work aims to understand the effects of particle size and density of MgO on the microstructures, and thermophysical and mechanical properties of the CPCMs. A eutectic carbonate salt of NaLiCO3 was used as the phase change material (PCM) with a melting temperature of around 500 degrees C and graphite flakes as the thermal conductivity enhancement material (TCEM). Two types of MgO were used in the work with one being light MgO and the other heavy MgO. The results demonstrated clear evidence of salt migration within the CPCMs made from both types of MgO during thermal cycling, leading to a more homogenous distribution of the salt as well as the CSM and TCEM. For a given particle size and TCEM loading, the light MgO (with a higher surface energy) gave a more considerable extent of particles migration and rearrangement than the heavy MgO (with a lower surface energy). Due to the higher surface energy, a much denser structure and better PCM containment of the CPCMs were observed with the light MgO formulations. For a given MgO type, smaller MgO particles yielded smaller internal pores and a more rigid structure, leading to a better containment of the PCM and more stable composite structure during thermal cycling. The microstructural characteristics observed were also found to be closely related to the thermophysical and mechanical properties of the CPCMs; a CPCM composite made with ingredients of a similar size gave a more compact structure and hence a better combination of thermal conductivity and mechanical strength.