Highly monodispersed colloidal silica particles in sizes ranging from 700 to 1300 nm were synthesized by a two-step method. The transmission electron microscope images showed that the mean standard deviation of these particles is less than 5%. Silica particle size range for self-assembly in the gravitational sedimentation was broadened through adjusting the viscosity of media in silica suspensions. The synthetic opals with particle diameters ranging from 700 to 1300 nm were obtained,and their optical characteristics were examined. Scanning electron microscope images showed that the synthetic opals have a face-centered-cubic structure and have partial photonic band-gaps. This provides the foundation for the study of complete photonic crystals with wavelengths within near-infrared and infrared regions and for the fabrication of other macroporous materials.