The crystal structure of AlN film deposited on a (100) surface of a sapphire by a DC reactive magnetron sputtering is studied by a X-ray diffraction, the diagram of the X-xay diffraction shows that the film made by this way is a single crystal film; using a Nd∶YAG laser with a repetition rate of 10 Hz,a pulse width of 10 ns, a maximum average power of 20 W and a maximum energy per pulse of 2 J as a pump. Its second harmonic generation is studied, some conclusions were analysed, and the results indicate that the AlN film deposited on a (100) surface of a sapphire can generate an effective second harmonic wave in a large range of incidence angle, and the generated power of the second harmonic wave is symmetric to the normal of the surface of the AlN film. This proves that the normal of the surface of the AlN film is the axis of the AlN.