Abstract Considering the low thermal conductivity of organic phase change materials (PCMs), a modification of Octanoic acid (OA) and Myristic acid (MA) binary mixture with a mass ratio of 87:13 was studied. The influence of expanded graphite (EG) and Hydroxylated multi-walled carbon nanotubes (MWCNTs) on the thermal conductivity of OA-MA were compared and analyzed. The morphology of these two nanoparticals and OA-MA/EG PCM were measured by scanning electronic microscope (SEM). 7 wt% EG was finally selected as the additive to modify the thermal conductivity of OA-MA. By differential scanning calorimeter (DSC), the phase transition temperature was measured to be 6.8 °C and the enthalpy of fusion was 136.3 J/g. The thermal conductivity of OA-MA/EG was 0.9975 W/(m·K) tested by Hot Disk, which remarkably increased by 235.75% compared with the thermal conductivity of OA-MA. Undergoing 100 freeze-thaw cycles, the properties of the OA-MA/EG did not change significantly, which shows the composite PCM is especially suitable for its application in pharmaceutical refrigerated transport system.