The photonic band gap of self-assembled thin film could be controlled precisely in this study using the seeded growth of spherical SiO2 nanoparticles. The monodispersed SiO2 nanoparticles were fabricated using the Stober process. The nanoparticle with the narrowest particle size distribution was used as the nucleation seed. The particle size could be controlled to between 50 nm and 1000 nm using the Stober process and seeded growth methods. When the seed content exceeded 0.1 wt% in solution, no new particles were observed and narrow size distribution was thus obtained. A self-assembled thin film was prepared on a glass substrate by evaporation with capillary extraction. A linear relationship was observed between the average particle size and the photonic band gap according to the transmittance and reflectance measurements. The variation of photonic band gap was well matched with the theoretical consideration of Bragg's diffraction law.