Polycrystalline Al2O3 substrates have been proposed and tested for high temperature micro devices packaging intended for operation at temperatures up to 500°C. The dielectric properties of this material, including dielectric constant and effective volume conductivity, at elevated temperatures are of interest, especially for RF packaging applications. This article reports temperature dependent dielectric properties of polycrystalline 96% Al2O3 substrates from room temperature to 550°C measured by the AC impedance method at 120 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. We observed negative temperature coefficients of volume electrical conductivity of 96% Al2O3 at 1 k, 10 k, and 100 kHz between room temperature and 50°C. The dielectric constant of the material increases significantly with temperature at frequencies below 10 kHz. The physical mechanisms of these dielectric behaviors of 96% Al2O3 at elevated temperatures are discussed.