Abstract: The observed growth habit of TiO₂, α-Al₂O₃ crystals observed under hydrothermal conditions were successfully explained by the coordination polyhedron rule concerning
growth habit. It is concluded that the habit of α-Al₂O₃ crystal is the hexagonal platy {0001}, and the relationship of growth rate of various faces is:
V_{0001}<V_{1123}<V_{0112}<V_{1120}<V_{1010}; the growth habit of rutile (TiO₂) crystal is elongated prismatic form, and the relationship
of growth rate of various faces is: V_<110><V_<100><V_<101><V_<001><V_<111>; the growth habit of anatase (TiO₂) crystal is
a tetrahedron, and the relationship of growth rate of various faces is: V_<010>=V_<001>>V_<010>>V_<111>. These results were compared with previous studies based on PBC theory.

Key words: TiO₂, α-Al₂O₃, growth habit, hydrothermal method