Effect of Mg doping on the properties of AlGaN layers gro wn on sapphire substrates by metal_organic chemical vapor deposition were studie d using x_ray diffraction and Raman scattering. When the doping of Mg was low, the E2 mode shifted to lower frequency and the full width half at maximum of the rocking curve and A1(LO) mode decreased. However, as the flow rate of CP2Mg increased ti ll the AlGaN was doped with a dose as high as 4×102/sup>cm-3, the quali ty of film de creased gradually and the E2mode shifted to higher frequency, indicat ing a compressive stress in the films. Finally, the relation between the Raman s hift and the stress alteration was described by Δσ＝-0298+0562ΔE.