Silaned nano-silicon dioxide (SiO₂) was used to improve the adhesion properties of carbon fiber/polyoxymethylene (CF/POM) composites. The CF and nano-SiO₂ were treated and the changes on the surface properties of the treated and untreated composites were studied by impact, three-point bending tests, and scanning electron microscopic analysis. The measurement showed that the fracture toughness increases with the increase of CF content, while very high content of CF did not further cause the increase of the toughness of CF/POM composite. The impact strength of treated specimens is still higher than those of the untreated ones. The modified composite with the good matrix/fiber adhesion possessed 20% higher interlaminar shear strengths compared to the composite having weak interface.