Mg-doped and undoped GaN epilayers were grown by metalorganic chemical vapor deposition on SiC substrates. The samples are both under tensile stress, while compared with undoped GaN, Mg doping would introduce much more defects and aggravate disorder so that the quality of film became worse. On the other hand, for the radius of Mg atom is larger than that of Ga, the compressive stress was introduced and tension in film decreased when Mg substituting Ga. Finally, we show as to the GaN:Mg epilayer, the quality of film would also influence the A₁(LO) mode besides free carriers.