The present invention is to produce an aluminum nitride powder which is turned into a sintered body at a temperature of not more than 1600.degree. C., thereby obtaining a sintered aluminum nitride in which the density and thermal conductivity are high and which can be properly used as a substrate material.Using a vapor phase reaction apparatus shown in FIG. 1, ammonia gas was fed from a reactor 2 heated at from 300 to 500.degree. C. and maintained at that temperature by a heating section 1 via a feeding tube 4 while being regulated by a flow regulator 3. At the same time, while being regulated by the flow regulator 5, nitrogen gas containing an organic aluminum compound is fed via a feeding tube 6 to obtain an aluminum nitride powder. The aluminum nitride powder is subjected to a heat treatment at from 1100 to 1500.degree. C. in a reducing gas atmosphere and/or an inert gas atmosphere to obtain an aggregate aluminum nitride powder. The aggregate aluminum nitride powder is subjected to a mechanical treatment to obtain an aluminum nitride powder in which the specific surface area value is not less than 30 m.sup.2/g and a ratio of an average particle diameter to a particle diameter in terms of the specific surface area is not more than 10.