An aluminum-nitride sintered body that has both high thermal conductivity and high mechanical strength, a fabricating method for the same, and a semiconductor substrate comprising the same. A material powder is prepared by mixing an aluminum-nitride powder, constituting 1 to 95 wt. %, having an average particle diameter of 1.0 &mgr;m or less obtained by chemical vapor deposition, with another type or types of aluminum-nitride powders constituting the remaining part. The material powder is sintered in a non-oxidizing atmosphere to obtain a sintered body having an average grain diameter of 2 &mgr;m or less and a half width of the diffraction peak on the (302) plane, obtained by X-ray diffraction, of 0.24 deg. or less. Formation of a metallized layer on the sintered body yields a semiconductor substrate.