The effects of sintering additives, microstructure, and morphology of second phase on the thermal conductivity of aluminum nitride ceramics are discussed in this review. The thermal conductivity of AlN is highly dependent on the types and amount of sintering additives, second phase morphology and microstructure. The morphology of second phase in AlN ceramics could be controlled by changing the cooling rate. The amount of second phase was able to be minimized by using the transient sintering additives. The thermal conductivity of AlN could be enhanced with the inclusion of the transient sintering additives. The thermal conductivity of AlN ceramics was evaluated by Raman spectroscopy. High temperature AC impedance studies were conducted to characterize the electrical conduction mechanism of AlN ceramics.