Three-dimensional self-assembly of colloidal particles by ice crystallization is observed in a water based-colloidal suspension. When water containing polystyrene beads freezes and is crystalized into ice, the polystyrene beads are extruded outward from the ice regions. Consequently, the concentration of polystyrene beads increases rapidly and they are assembled together into a regular structure. As ice crystallization proceeds, a color appears abruptly. This indicates that the polystyrene beads have been assembled three dimensionally. The generated three-dimensional structure is confirmed by scanning electron microscopy images and the existance of a photonic band gap measured by an ultraviolet-visible spectrophotometer. The sample fabricated by this method is compared with a sample made by conventional vertical deposition. (C) 2002 American Institute of Physics.