The unmodified and vinyl trimethoxysilane (VTS) modified nano SiO
2 and micro SiO
2 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-SiO
2/PP composites. The effects of the SiO
2 particle size, dispersibility and interfacial modification on the reinforcement of the WF-SiO
2/PP composites were determined. Infra-red spectra show that SiO
2 is grafted onto the PP matrix. Compared with the unfilled WF/PP composites, the resulting WF-SiO
2/PP composites incorporated with 9% micro SiO
2 or 9% nano SiO
2 (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 SiO
2 and nano SiO
2, respectively. The creep resistance is also improved obviously via wet dispersion process and VTS modification. The impact strength of the WF-SiO
2/PP composites increases by 17% by filling the micro SiO
2 via dry or wet dispersed modes. This study demonstrates that adding proper amount of nano or micro SiO
2 can increase the impact strenth of WF-SiO
2/PP composites by 15%-25% on the premise of a uniform dispersion and establishment of a well-bonded interface between the SiO
2 and the matrix.