Abstract: Polyoxymethylene (POM)/silica nanoparticle composites were fabricated by in-situ polymerization using silica nanoparticles surface-modified with epoxy (RNSE) and methyl groups (DNS) as fillers, and their structure, thermal stability and crystallization behaviors were investigated by scanning electron microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry and polarized light microscopy. The results indicated that RNSE was well dispersed in the POM matrix at the content lower than 1 phr. RNSE exhibited a better dispersibility in the matrix than DNS. The silica nanoparticles could promote the crystallization of POM, thus resulting in the decrease of spherulite size and the improvement of crystalline degree. Compared with DNS, RNSE presented a more effective enhancement on the crystallinity of POM. However, the thermal stability of the composites was reduced by the silica nanoparticles.