Liquid-phase sintering of aluminum nitride (AlN) with additives was reviewed. The most important innovation was the discovery of critical sintering aids for AlN densification, specifically rare-earth compounds and alkali-earth compounds. These additives are extremely valuable for increasing thermal conductivity by trapping and removing oxygen in the AlN lattice during firing. Consequently, thermal conductivities in AlN ceramics of 100 to 260W/mK were developed. We also studied the effects of parameters such as raw powder, additives, composition, and firing condition in liquid-phase sintering with AlN-sintering aids, focusing on oxygen impurities in the system. The sintering behavior of powder compacts was investigated by evaluating the densification, the lattice constant c for AlN, and the dihedral angle of the interface between the AlN grains and the grain boundary liquid-phase. In our results, the change in densification was closely related to changes in the lattice constant c and the dihedral angle. That is, the sintered density increased with an increase in the oxygen dissolved in the AlN grains and with the improvement in wettability between the solid and liquid phase.