In the process of PVT growth of AlN crystals, there is difficult to maintain ideal thermodynamic equilibrium conditions, causing crystal defects being inevitably generated. High temperature annealing technology has received much attention due to their effectiveness in improving crystal integrity. In this paper, AlN samples grown by PVT method were annealed at high temperature in N₂ atmosphere. In order to evaluate the crystalline quality and structural perfection of AlN before and after thermal annealing, high-resolution X-ray diffraction (HRXRD) and Raman spectrum were carried out. In addition, the impurity related band gap changes in the optical properties of AlN crystals were characterized by room temperature photoluminescence (PL) and absorption spectra. The crystal quality of these AlN crystals was significantly improved after annealing at 1400-1800 ℃. The full width at half maximum (FWHM) of the (10¯12) plane X-ray rocking curve decreased from 104.04 to 79.92 arcsec (1 arcsec=0.01592°) after annealing at 1400 ℃. As the annealing temperature increases, the absorption was significantly enhanced and the band gap became larger, indicating that the annealing process was beneficial to improve the quality of AlN crystals. The results of secondary ion mass spectrometry (SIMS) demonstrate that the annealing process reduces the C impurity, resulting in an increase in band gap of AlN crystal, which is consistent with the results of optical absorption.