The surface phonon polariton (SPP) mode of aluminum nitride (AlN) thin film grown on sapphire substrate is investigated experimentally and theoretically. For the experimental study, p-polarized infrared attenuated total reflection measurements are performed. A strong absorption dip which corresponds to the SPP mode of AlN thin film is clearly observed at 839.0cm-1. For the theoretical study, the surface-polariton dispersion curve of the AlN thin film has been simulated. Finally, the experimental and theoretical values of the SPP mode are compared. The results revealed that the uncertainty between the theoretical and experimental SPP values is less than 2%.