The aim of this study is to improve the dielectric and mechanical properties of HDPE/BaTiO3 composites by binary BaTiO3 particles, when the volume fraction of BaTiO3 is constant. In this study, it was found that the pack density of binary BaTiO3 particles in HDPE/BaTiO3 composite relies on particle ratio and volume fraction of small particles. It is found that the addition of 50 vol% 1600nm BaTiO3 particles can boost the dielectric constant of HDPE control from 2 to 30 (14 times higher) at 40 Hz and 19 (8.5 times higher) at 40 MHz, respectively. When the particle ratio was 4, the substitution of 10 vol% 1600nm BaTiO3 particles by 10 vol% 400nm BaTiO3 particles can further enhance the dielectric constant of HDPE/L-BT (10/10) from 30 to 50 (67% increase) at 40 Hz and from 19 to 42 (121% increase) at 40MHz, respectively, without greatly influencing the volume resistivity of HDPE composites. In addition, the thermal conductivity of HDPE with binary BaTiO3 particles were all above 2.0 W/(m center dot K). (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1101-1108