This article reports temperature dependent dielectric properties of polycrystalline 96% Al2O3 substrates from room temperature to 550°C measured by the AC impedance method from 120 Hz to 1 MHz.  Negative coefficients of volume electrical conductivity between 1 kHz and 100 kHz at room temperature to 50°C and dielectric constant increases with temperature below 10 kHz were found.  The physical mechanisms of these behaviors is discussed.