Abstract: The unmodified and vinyl trimethoxysilane (VTS) modified nano SiO₂ and micro SiO₂ were used as the reinforcement phases to incorporate into polypropylene (PP) by direct dispersion (dry dispersion) or by dispersion in solution (wet dispersion), respectively. The wood flour (WF) was used as modifier to prepare WF-SiO₂/PP composites. The effects of the SiO₂ particle size, dispersibility and interfacial modification on the reinforcement of the WF-SiO₂/PP composites were determined. Infra-red spectra show that SiO₂ is grafted onto the PP matrix. Compared with the unfilled WF/PP composites, the resulting WF-SiO₂/PP composites incorporated with 9% micro SiO₂ or 9% nano SiO₂ (mass ratio) via dry dispersion mode exhibit 21% and 18% reduction in the flexural strength, respectively. However, wet dispersion and VTS modification result in 17% and 22% increase in flexural modulus for incorporating the micro SiO₂ and nano SiO₂, respectively. The creep resistance is also improved obviously via wet dispersion process and VTS modification. The impact strength of the WF-SiO₂/PP composites increases by 17% by filling the micro SiO₂ via dry or wet dispersed modes. This study demonstrates that adding proper amount of nano or micro SiO₂ can increase the impact strenth of WF-SiO₂/PP composites by 15%-25% on the premise of a uniform dispersion and establishment of a well-bonded interface between the SiO₂ and the matrix.