A series of monomer casting (MC) nylon-6/SiO2 composites were prepared via in situ polymerization. It was found that the tensile strength, storage modulus, and notched charpy impact strength of the composites were improved and reached maximum at 3–5 wt% loading of SiO2. The α relaxation peak corresponding to the glass transition temperature (Tg) shifted to high temperature with increasing SiO2 content. Addition of SiO2 led to an increase of the melting and crystallization temperatures, and crystallinity. It also reduced the induction time of crystallization, advance the crystallization process of MC nylon-6, and improve the crystal growth rate. The self-nucleation crystallization analysis indicated that SiO2 particles played the role of facilitating the crystallization of the matrix mainly via accelerating the generation of crystal nucleus. By addition of SiO2 particles, the fracture surface of MC nylon-6 changed to distant striations with many yield folds accompanied by a large number of stress whitening, displaying much obvious character of tough fracture. SiO2 particles can be pulled-out under stress by being covered with MC nylon-6 resin due to strong interfacial interaction and presented a skin–core structure. © 2012 Society of Plastics Engineers.