Fe₃O₄ was prepared on the TiO₂-coated natural mica substrate. The natural mica has an average particle size of 22 ㎛. The substrate was coated on TiO₂ thin films using hydrothermal synthesis at pH 1.5-2.5 at 75℃. The Fe precursor solution was prepared by mixing FeSO₄ (for Fe²⁺ ion) and FeCl₃ (for Fe³⁺ ions) with different molar ratios such as 1/2, 1/1, 2/1, 3/0, and Fe₃O₄ only. X-ray diffraction analysis shows that the crystal structure depends on the FeCl₃-to-FeSO₄ molar ratio. Fe₃O₄ crystal phase could be obtained at higher FeSO₄ contents.