The Brillouin scattering of lithium niobate-tantalate crystals (LiTaNb1-cO3, x-0.0013, 0.0062, 0.015, 0.06) has been investigated at room temperature. The elastic stiffness constants and piezoelectric stress constants of these crystals have been obtained. The temperature dependence of the Brillouin frequency shift for one of these crystals in the range from 50 K to 580 K has been measured. An abnormal peak has been observed in the Brillouin spectra for some phonons in addition to the three normal peaks.