Abstract: Three kinds of activated carbon were prepared from rice husk with H₃PO₄, KOH, ZnCl₂ as activating agents at 600℃. The biochar and three kinds of activated carbon were used to reinforce high density polyethylene (HDPE) to prepare biochar/HDPE composites and activated carbon/HDPE composites. The mechanical properties of the composites were tested and analyzed. The results show that higher specific surface area and more developed pore structure are obtained in activated carbon than in biochar, and better mechanical properties are obtained in activated carbon/HDPE composites than biochar/HDPE composites. Besides, the highest specific surface area of 714.27m²/g is obtained in activated carbon which is activated by H₃PO₄ (activated carbon(H₃PO₄)), and better comprehensive mechanical properties are obtained for activated carbon(H₃PO₄)/HDPE composite due to its excellent flexural properties, tensile properties, rigidity, elasticity, creep resistance and anti-stress relaxation ability. The flexural strength, flexural modulus, tensile strength and tensile modulus of the activated carbon(H₃PO₄)/HDPE composite are 38.66 MPa, 2.46 GPa, 32.17 MPa and 1.95 GPa, respectively. This study can provide useful experience for utilization of activated carbon in composites.