The phonon-dispersion relations of LaGaO3 crystal are calculated using the first-principle calculations. The first-order Raman spectra in the LaGaO3 were measured in the temperature range 20-690K. The temperature dependence of linewidth of several modes [the Ag(57 cm-1) and the B2g(118 cm-1) for the low-temperature phase (LT), and the Eg(52 cm-1) and the Eg(160 cm-1) for the high-temperature phase (HT)] was analyzed using TDOS due to cubic term. The calculated results reproduce the observed ones in this temperature range without the temperature range (350-400 K) on the precursor effect of the phase transition. Temperature dependence of the linewidth for some rotational modes [the Ag(57 cm-1) and the B2g(118 cm-1) for the LT, and the Eg(52 cm-1) for the HT] is dominated by the scattering of a thermal phonon by the observed phonon, while that for the bending mode [the Eg(160 cm-1) for the HT] 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 large frequency gap in the DOS that produces a difference in anharmonic effects on each peak's channels in the HT.