Amorphous TiO2 particles were prepared by the various reverse systems such as reverse micelle, reverse microemulsion, reverse emulsion, and by the sol-gel method, and investigated on their photocatalytic properties. After calcinating for 3 h at 200 degrees C, we found that they indicated two kinds of band-gap energies and gave the photocatalytic activity with irradiation of either UV or visible light, while anatase state did not work with visible light. However, any amorphous states were gradually converted into crystalline anatase with continuous irradiation, even without heating.