In this study, three grades of diatomite particles, DPI, DP2 and DP3, were used to produce phase change material composites, DPI P, DP2P and DP3P, by absorbing paraffin into the pores of diatomite particles. The absorption mechanism of the finest DPI was found to be different with that of DP2 and DP3. In addition to absorption mechanism, stability of the composites was also investigated. However, a significant amount, up to 67%, of paraffin leaked from the composites when they were used in concrete mixture directly. In order to prevent the leakage, a surface-modification method was proposed and promising results were achieved. Thermal behaviors of concrete panels incorporating with DP1P and Micronal were then compared with a control one. The results indicate that addition of PCM composites improves the thermal inertia. (C) 2014 Elsevier B.V. All rights reserved.