The lattice vibrations of diluted magnetic semiconductor GaMnN epitaxial films grown on sapphire substrates by metalorganic chemical vapor deposition have been investigated using infrared reflectance spectroscopy. The revised genetic algorithm was successfully used to extract the Lorentz oscillator model parameters of GaMnN from infrared reflectance spectra. Comparing the Lorentz oscillator model parameters of GaMnN with those of GaN, it was found that ωTO moved to higher frequencies, γ, ε∞ and εs increased, but the ωLO was almost not changed. The effects of Mn on the properties of lattice vibration and the mechanisms of variations of the dielectric function were also analyzed and discussed in this paper.