Microwave energy was used to sinter high thermal conductivity AlN ceramics (160–225 W/mK). The effects of sintering time, temperature, and amount of additive on phase composition, phase distribution, densification behavior, grain growth, and thermal conductivity were studied. The thermal conductivity of AlN was greatly improved by the addition of Y2O3, extended sintering time, and higher sintering temperatures. Thermal conductivity development in Y2O3-doped AlN showed two distinctive time regimes: (i) densification, where full densification, secondary phase formation, concentration and segregation, and rapid purification of AlN grains occur, accompanied by a large increase in thermal conductivity; (ii) postdensification, where grain growth and secondary phase sublimation/evaporation occur, yielding a further increase in thermal conductivity. Our results indicate that microwave sintering is a promising approach for synthesis of high thermal conductivity AlN ceramics.