The N doped SiO2 thin films were prepared by radio frequency magnetron reaction sputtering technique. It is found that the N doped SiO2 thin films have ferromagnetism. Magnetic order is easy to form in the system in which silicon nitride particles with relatively small sizes are distributed uniformly in silicon oxide matrix. When the substrate temperature of the film was 400℃，the N doped SiO2 thin film possesses the largest saturation magnetization and coercivity，being 35 emu/cm3 and 75 Oe，respectively. The magnetism of the film may originate from the interfaces between silicon nitride and silicon oxide. Calculations based on the first principles show that net spins exist in the N doped SiO2 film. The orbit magnetic moments caused by the charge transfer through the interface between silicon nitride and silicon oxide also contribute to the ferromagnetism of the film.