The paper reports the use of Y O and La O co-doping as a composite sintering additive for the fabrication of γ-AlON transparent ceramics by pressureless sintering. The Y enhanced the mobility of grain boundary and accelerated the grain growth, whereas the La has the opposite effect on grain growth. The sintering additives could cause the formation of liquid phase during sintering, which would greatly promote the densification and eliminate pores. High transparent AlON ceramics with the in-line transmittance of 80.3% at 400nm wavelength have been prepared when the concentration of sintering additives was 0.12wt% Y O and 0.09wt% La O .