The effect of annealing on tan delta and microstructures of aluminum nitride (AlN) ceramics were explored. Yttria was added as a sintering additive to AlN powders, and the powders were pressureless-sintered at 1900 degrees C for 2 h in a nitrogen flow atmosphere. In succession to sintering, AlN samples were annealed at 720, 970 and 1210 degrees C for 2 and 4 h. Very low tan delta values between 2.6 and 6.0 x 10(-4) at 28 GHz were obtained when the AlN samples were annealed for 4 h at all the annealing temperatures.