Aluminum oxynitride (AlON) ceramic was synthesized by one-step pressureless sintering technology using low cost and easily available α-Al2O3 and aluminum powders as raw materials. The sintering temperature was reduced because aluminum powders were nitridized into high activity AlN under the flowing nitrogen atmosphere. The curves of thermal analysis, microstructure and atomic distribution were investigated. The influence of sintering temperatures on phase composition, sintering densification and flexural strength was also explored. The experimental results showed that α-Al2O3 and aluminum powders were acceptable substitutes for more expensive AlN ultrafine powders. Under the optimum sintering process at 1750 °C for 2 h, the sintered density and flexural strength of AlON ceramic were higher, 3.62 g/cm3 and 321 MPa, respectively. The sintering temperature was decreased by 50 °C because newly formed high activity AlN in situ reacted with Al2O3 into Al23O27N5, enhancing flexural strength by 29.4%. However, the sintering temperature could not be too high because grain growth and displacement of oxygen atoms from AlON ceramics by nitrogen atoms caused a decline in sintering densification and flexural strength.