The aim of the experimental study is to determine thermal conductivity of composites as a function of volume fraction and size of aluminum (Al) particles. High density polyethylene (HDPE) were filled with Al particles that have different particle sizes, 80 nm and 40-80 μm. Nanocomposites were prepared by the melt-mixing technique at various volume fractions (up to 33%). Thermal conductivity of polymer composites has been measured by C-Therm thermal analyzer depending on the modified transient plane source technique. Thermal conductivity of HDPE/Al composites increases by increasing volume fraction of Al in HDPE matrix. It is found that size of Al particles hasn’t significant effect on thermal conductivity, thermal conductivity of HDPE/Al (80 nm) is close to thermal conductivity of HDPE/Al (40-80 μm).