Properties of some GaN and GaN:Mg films with different blue luminescence relative intensity were investigated by Rutherford backscattermg/channeling measurement, double crystal X-ray diffraction measurement, photoluminescence technique, respectively. The blue luminescence in unintentional doped GaN layers obvious relates with the crystalline quality of those films. The RBS/channeling Xmin and the FWHM of double crystal X-ray diffraction increase with the increase of the intensity ratio of the blue luminescence to the band edge emission. The blue luminescence is due to some defects in unintentional doped GaN films. The peak position of blue luminescence in unintentional doped GaN films do not shift and its intensity superlinear increases with the growth of excitation density. These results indicate that it is the transition from the free electron to acceptor levels. The peak energy of blue luminescence about 2.88 eV in GaN:Mg films increases about 25meV and its intensity linearly increases with the growth of excitation density.These results indicate that it is the transition of donor-acceptor pair in GaN:Mg films.The mechanism of the blue luminescence in unintentional doped GaN films is different from the blue luminescence about 2.9eV in GaN:Mg films.