Abstract. The influences of filler size and content on the properties (thermal conductivity, impact strength and tensile strength) of Al 2 O 3 /high density polyethylene (HDPE) composites are studied. Thermal conductivity and tensile strength of the composites increase with the decrease of particle size. The dependence of impact strength on the particle size is more complicated. The SEM micrographs of the fracture surface show that Al 2 O 3 with small particle size is generally more efficient for the enhancement of the impact strength, while the 100 nm particles prone to aggregation due to their high surface energy deteriorate the impact strength. Composite filled with Al 2 O 3 of 0.5 µm at content of 25 vol% show the best synthetic properties. It is suggested that the addition of nano-Al 2 O 3 to HDPE would lead to good performance once suitably dispersed.