The electrical conductivity of CaF 2 -doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF 2 -doped AlN ceramics were sintered with a second additive, Al 2 O 3 , added to control the electrical conductivity. The effects of calcium fluoride (CaF 2 ) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF 2 additive have no secondary phases at grain boundaries. Addition of Al 2 O 3 caused the formation of amorphous phases at grain boundaries. Addition of Al 2 O 3 to CaF 2 -doped AlN ceramics at temperatures 200°C-500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al 2 O 3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.