The post-treatments including ultrasonic dispersion, freeze-drying and ball milling were used to reduce the particle size of Aluminum oxynitride (AlON) powders, which were synthesized by the carbothermal reduction and nitriding process. Effects of post-treatment methods and time on the microstructure and particle size distribution were systematically evaluated. Results show that ball milling for 12 h can effectively break up some large particles caused by agglomeration and reduce the particle size from 19.61 μm to 1.347 μm. The preparation of AlON transparent ceramics was explored preliminarily. The AlON powders with 0.4 wt% Y2O3 and 0.02 wt% La2O3 were used to prepare the green body by dry pressing and cold isostatic pressing. Then the ceramic green body was pressless sintered with flowing nitrogen atmosphere at 1875℃ for 24 h, 6 h and 3 h respectively. When the sintering schedule was 3 h heat preservation, the density of AlON ceramic has been greatly improved.