Abstract: Nanocrystalline cellulose (NCC) was prepared from hydrolysis of microcrystalline cellulose (MCC) using sulfuric acid. NCC can be used as filler partialy to replace SiO₂ and utilized in the preparation of (NCC-SiO₂)/NR(natural rubber) composites. The results show that the composites can not only maintain basic mechanical properties, but also decrease heat build-up from 19.9 ℃to 10.6 ℃ and permanent set from 11.4% to 5.9%. When the mass ratio was added with NCC∶SiO₂=10∶20, the grade one and six flex cracking times of (NCC-SiO₂)/NR composite material increase from 2.5×10⁴ and 6.0×10⁴ (filled by SiO₂) to 7.0×10⁴ and 1.2×10⁵, respectively. Aging performance is also superior to SiO₂/NR composite materials. After hot air aging, the tensile strength, tear strength and hardness of the composite materials after aging increase by 40%, 21% and 25%, respectively, and the permanent deformation droppes by 25%, while tensile strength and elongation at break remain basically unchanged. Dynamic mechanical performance test shows that when the mass ratio of NCC to SiO₂ is from 0∶30 to 20∶10, the rolling resistance of rubber tanδ at 0 ℃ keep almost constant, but the skid resistance of rubber tanδ at 60 ℃ droppes from 0.060 to 0.049. The results of SEM photographs prove that better dispersion and stronger interfacial interaction were achieved in NR matrix filled by NCC-SiO₂ than by SiO₂.