In this work, diamond particle (DP), based on its excellent heat transfer property as well as electric insulativity, was adopted to enhance the thermal conductivity (k) and volume resistivity (ρ)of organic shape stabilized phase change material (SSPCM) for the first time. Using the method of in situ polymerization, PMMA serving as the supporting material and PEG acting as the PCM were compounded with the DP. Results show that the DP is physically combined with PMMA/PEG matrix, and all the samples remain solid without leakage above the melting point of the PEG. The k and ρ of the composite SSPCM obviously increased with the increase of DP content. When the mass fraction of DP is increased to 30%, the k and ρ of composite achieved 0.454 W•(m•K)-1 and 6.12×1010 Ω•cm, respectively. It is also indicated that all the specimens provide available thermal energy storage density and thermal stability.