Far-infrared (FIR) reflection measurements have been employed to study the long-wavelength optical phonon properties in cubic MgZnO thin films grown by reactive electron beam evaporation on sapphire substrates. The frequencies of the optical phonons with T2 symmetry are obtained by calculating the FIR reflection spectra using a multioscillator model and Kramers–Kronig dispersion analysis. The modified random-element-isodisplacement model has been used to analyze the long-wavelength optical mode behavior in both cubic and hexagonal MgZnO alloys. It is found that cubic MgZnO exhibits a two-mode behavior, while for hexagonal MgZnO, the E1 mode displays a two-mode behavior and the A1 mode a one-mode behavior. The effects of structural transition on the optical phonon frequency, optical mode behavior, and force constant have also been clearly revealed.