Normal vibrational modes of the [B3O6](3-) metaborate ring are obtained at the Hartree-Fock level by ab initio calculations. The Raman and IR spectra are predicted, and the former are compared with available experimental results of molten crystal metaborates that contain [B3O6](3-) rings, i.e., beta-BaB2O4 and CsBO2 crystals. The internal vibrations of the [B3O6](3-) ring are dominant in the whole crystal lattice vibrations of these nonlinear optical crystals. Also, the [B3O6](3-) ring accounts for the IR transparency cut-off in such crystals. It is deduced from the predicted IR spectrum of a [B3O6](3-) ring that the transparency cut-off frequency of the beta-BaB2O4 crystal on the IR side is below 4000 cm(-1) (>2.5 mu m), which agrees well with the measured data. This study is also useful for understanding the vibrational contributions to nonlinear optical polarizabilities.