Raman scattering spectra of GaN and Mg-doped GaN films grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire substrate are investigated. Some weak peaks are also observed in two spectra besides the allowed E

2

and A

1

(LO) phonon modes in

Z

(X-)

Z

backscattering geometry. The origins of these peaks are discussed and the differences between two spectra are compared. Two peaks at 640 and 660cm

-1

are observed both in the spectra of GaN and Mg-doped GaN samples. The 640cm

-1

peak is attributed to an overtone process of the highest acoustic-phonon branch at the zone boundary (L point). The peak at 660cm

-1

is attributed to the optical-phonon branch at the zone boundary or the local vibrational mode induced by defect. In the spectrum of Mg-doped GaN sample there is a broad feature near the two peaks. It is suggested that the broad feature arises from the local vibrational mode of Mg-N bond which superposed on the two peaks mentioned above or arises from defect-induced broaden. Two additional peaks at 276cm

-1

and 376cm

-1

are also observed in the spectrum of Mg-doped GaN sample. The peak at 276cm

-1

is the local vibrational mode of Mg-N bond and the peak at 376cm

-1

is the local vibrational mode of impurity-defect complex. The frequency of E

2

mode in Mg-doped GaN shifted to low energy side compared with GaN sample

and located at 568cm

-1

at room temperature. This indicated that the relaxation of residual stress existed in Mg-doped sample. The stress relaxation is contributed to Mg-induced generation of dislocation. Furthermore

electron-phonon interaction also can induce frequency shift of E

2

mode.