  The role of orbital ordering on metal-insulator transition of
transition-metal oxides is investigated by the cluster self-consistent field
approach in the strong correlation regime. A clear dependence of the insulating
gap on the orbital order parameter is found in the single-particle excitation
spectra. The thermal fluctuation drives the orbital order-disorder transition,
diminishes the gap and leads to the metal-insulator transition. The interplay
between spins and orbits results in unusual temperature dependence of the
orbital polarization in the orbital insulator, which can be seen in the
resonant x-ray scattering intensity.
