The synthesis and sintering behavior of AlON from a mixture of Al₂O₃-AlN by the reactive plasma treatment were investigated. AlN, Al₂O₃, Y₂O₃ and CeO₂ powders were used as starting materials. These powders were mixed and formed into a pellet. The pellet was sintered in the reactive Ar-N₂ plasma generated inductively in a quartz tube by using an r.f. generator (4MHz, 5kW or 10kW) and a work coil. High AlON content sample was obtained by 30min sintering of mixture with a composition which corresponds to the AlON single phase region, in the sintered body near the r.f. coil for a short time duration (5min). When a large amount of Al₂O₃ was added into the starting powder, a dense sample was obtained. At a low plate output (5.5kW), the density decreased with increase in distance between the sample and the lower end of the work coil. At a high plate output (7.7kW) of the r.f. generator, the density of the sintered sample near the coil decreased, because the plasma power was too high to sinter the sample without causing decomposition. At a higher plate output (10.0kW), similar tendency was observed in the sintered sample near the coil as to various sintering durations. At a low position which was far from the coil, the density reached a maximum value in 3min (10.0cm) and 20min (13.5cm) sintering. Therefore it seemed that the optimum sample position was existed. The linear shrinkage also increased to a maximum value at this position. From the SEM observation, the grain growth was observed after 30min sintering, but the grain size was several micrometer smaller than that by the usual sintering method. The density of the sintered sample using Y₂O₃ was higher than that using CeO₂ or without any additives. Nitrogen or hydrogen addition to Ar plasma was effective to promote sintering, because the enthalpy of plasma and heat transfer from plasma to the sample were remarkably increased.