We present a theory of the collective orbital excitation termed orbital wave in perovskite titanates and vanadates with the triply degenerate t_2g orbitals. The dispersion relations of the orbital waves for the orbital ordered LaVO_3, YVO_3 and YTiO_3 are examined in the effective spin-orbital coupled Hamiltonians associated with the Jahn-Teller type couplings. We propose possible scattering processes for the Raman and inelastic neutron scatterings from the orbital wave and calculate the

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... ing spectra for titanates and vanadates. It is found that both the excitation spectra and the observation methods of the orbital wave are distinct qualitatively from those for the e_g orbital ordered systems.