Sintering aid was very crucial to influence the microstructure and thus the optical property of the sintered optical ceramics. The purpose of this work was to explain the difference between the sintering aids of Lisup.+ and Ysup.3+ on Alsub.23Osub.27Nsub.5 (AlON) ceramic via reaction sintering method. The effects of LiAlsub.5Osub.8 (LA) and Ysub.2Osub.3 on the sintering of Alsub.2Osub.3-AlN system were carefully compared, in terms of X-ray diffraction (XRD), microstructure, density, X-ray photoelectron spectroscopy (XPS) and optical transmittance. According to the XPS and XRD lattice analysis, the chemical structure of the materials was not obviously affected by different dopants. We firstly reported that, there was obvious volume expansion in the Ysup.3+ dopped AlON ceramics, which was responsible for the low transparency of the ceramics. Obvious enhancements were achieved using Lisup.+ aids from the results that Li: AlONs showing a higher transparency and less optical defects. A higher LA content (20 wt%) was effective to remove pores and thus obtain a higher transmittance (~86.8% at ~3.5 μm). Thus, pores were the main contributions to the property difference between the dopant samples. The importance of sintering aids should be carefully realized for the reaction sintering fabrication of AlON based ceramics towards high transparency.