Aluminum oxynitride powders were synthesized by carbothermal reduction, using nano-sized alpha-alumina, gamma-alumina and carbon as raw materials. Effects of the weight percentages of alpha-alumina in alumina (R) and synthesizing temperatures (T) on the preparation of single-phase AlON powders have been studied. The results showed that proper R can reduce the shrinkage and increase the formation rate of AlON phase, but a high R will reduce the formation rate of AlON phase, because the activity of alpha-alumina was lower than that of gamma-alumina. Single-phase AlON powder with excellent properties can be obtained under the condition of R=0.15, T=1700°C. Moreover, this single-phase AlON powder was well-dispersed with an average size of about 2 μm.