This paper deals with the preparation, characterization, thermal properties and thermal reliability of form-stable composite phase change materials (PCMs), composed of n-octadecane, expanded vermiculite, and perlite for thermal energy storage. The composite PCMs were prepared by incorporation of liquid n-octadecane within the expanded vermiculite (eVMT) and expanded perlite (ePLT), using a vacuum impregnation method. The microstructures of n-octadecane/expanded vermiculite and pearlite were characterized by scanning electron microscopy (SEM). Analysis of Fourier transform infrared spectroscopy (FT-IR) of the prepared composite PCMs showed good compatibility between n-octadecane and the expanded vermiculite and pearlite. The thermal conductivities of composites were reduced, based on the TCi results. Differential scanning calorimetry (DSC) analysis indicated that the n-octadecane/eVMT and n-odtadecane/ePLT composites maintained their large latent heat capacity and original phase change temperatures, due to large surface area and good dispersion of the eVMT and ePLT. TGA analysis revealed that the prepared composite PCMs had good thermal durability in the working temperature ranges. Therefore, n-octadecane based composite PCMs can be considered as suitable candidates for latent heat thermal energy storage, with high thermal performance.