The phonon-dispersion relations of LaAlO3 crystal are calculated using the first-principle calculations. The first-order Raman spectra in the LaGaO3 were measured in the temperature range of 20-300K for the rhombohedral phase. The temperature dependence of the linewidth of the A lg (132 cm-1) and that of Eg (489 cm -1) were analyzed using temperature-weighted two-phonon density of states (TDOS) due to the cubic term. The calculated results reproduce the observed ones in this temperature range. The scattering of a thermal phonon by observed phonons dominates phonon-phonon interaction on the rotational modes [the Alg (132 cm-1)]. Phonon-phonon interaction on the bending mode [the Eg (489 cm-1)] is dominated by both the scattering and the decay of the observed phonons into two phonons having lower energies. We found that this originates in the discrete spectra on the higher-frequency side in the phonon density of states that produces a difference in anharmonic effects on each peak's channels in the HT. This result is similar to that in the high-temperature phase in the case of LaGaO3 .