An aluminum nitride (AIN) ceramic with a thermal conductivity value of 272 W.(m.K)(-1), which is as high as the experimentally measured thermal conductivity of an AIN single crystal, was successfully fabricated by firing at 1900degreesC with a sintering aid of 1 mol% Y2O3 under a reducing N-2 atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AIN grains.