Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy has been employed to investigate the exciton-phonon interaction in AlN. Longitudinal optical (LO) phonon replicas of free exciton recombination lines were observed in PL emission spectra, revealing the coupling of excitons with LO phonons. We have quantified such interaction by measuring Huang-Rhys factor based on polarization resolved DUV PL measurements. It was observed that the exciton-phonon coupling strength in AlN depends on the polarization configuration and is much larger in the direction with the electrical field (E→) of the emitted light perpendicular to the wurtzite c -axis (E→ ⊥ c→) than in the direction of E→ ∥ c→. Furthermore, a larger coupling constant was also measured in AlN than in GaN. The large effective hole to electron mass ratio in AlN, especially in the E→ ⊥ c→ configuration, mainly accounts for the observed results.