Sintered bodies of aluminum nitride are produced by adding, as a sintering aid, an oxide mixture consisting essentially of calcium oxide, yttrium oxide and one or more oxides of tungsten oxide and molybdenum oxide or a mixture of compounds (precursors) capable of being converted into said oxide mixture by firing to aluminum nitride as a main component in such proportions that total amounts of the sintering aid is in the range 0.1 to 20% by weight (calculated as oxides of CaO, Y.sub.2 O.sub.3, WO.sub.3 and MoO.sub.3) based on the sum of the weight as the oxide forms of the sintering aid and aluminum nitride, molding the resulting composition and sintering the molded composition. The composition can be sintered at relatively low temperature. The black sintered bodies thus obtained are especially useful as materials for insulating substrate, heat sink, packaging for semiconductor device, etc. due to their high density and good thermal conductivity.