The intermittent nature of solar energy requires integration of an energy storage system with solar collector devices in order to have un-interrupted supply of energy in the absence of the availability of solar energy. The use of Phase Change Material (PCM) and rock bed for heat storage has been studied in the past decades. However, limited knowledge exists on the use of the combination of PCM and rock particles for high temperature heat storage with air as the medium of heat transfer. The aim of this paper is to show experimentally by means of comparison, the thermal behaviours of thermal energy storage with only rocks and that with rocks and PCM. A heat storage consisting of only rocks and that with rocks and PCM has been designed and built. The PCM material used is a mixture of NaNO3 and KNO3 in the ratio of 60:40 percent (mol%). The charging and thermal de-stratification profiles were investigated for both cases. The results showed that the introduction of PCM cylinders in a bed of rocks for heat storage does not only increase the energy content of the storage but it also increases the vertical thermal conductivity. The increase in vertical thermal conductivity is attributed to the high thermal conductivity of the PCM container material. (C) 2013 Elsevier Ltd. All rights reserved.