Abstract: A series of composite phase change materials (PCMs) were prepared by using solid paraffin and lowdensity polyethylene (PE-LD) as raw materials, diatomite/expanded graphite modified with vinyl triethoxysilane as fillers and dicumyl peroxide (DCP) as a crosslinking agent. The effect of contents of paraffin and modified diatomite/expanded graphite on thermal stability and thermal conductivity of the composite PCMs were studied, and the effects of DCP content on their microstructure and mechanical properties were also discussed. The results indicated that the composite PCMs exhibited a good thermal stability at the paraffin loading of 60 wt%, and they also presented proper phase-change latent heat and phase-change temperatures. The addition of expanded graphite and diatomite improved the thermal stability of composite PCM, and an optimum thermal stability was achieved at a content of 4.6 wt%. The thermal enhancement effect was better from expanded graphite than from diatomite.; The addition of 4.6 wt% expanded graphite made the composite PCMs an improvement in thermal conductivity by 384.38 %. The mechanical properties of composite PCMs were enhanced with the addition of DCP, and the composite PCMs achieved optimum overall properties when 3 wt% DCP was added.