Thick translucent and luminescent Gd-alpha-sialon ceramic disks (0.7-1.06 mm in thickness) were prepared by hot pressing. The effect of carbon atmosphere on their optical properties during sintering was explored by change packing methods. The results show that the sample with a lower carbon contamination has a higher translucence in the visible band and IR band (450-3500 nm), increasing transmission around 10% even if it is thicker. When excited at 350 nm, Gd-alpha-sialon with the lower carbon contamination can produce a visible light at 450-500 nm bands, but the luminescence is very weak in the sample containing more carbon contamination. These indicate that carbon contamination causes a severe degradation of the optical properties of alpha-sialon ceramics, and reduction of carbon contamination of alpha-sialon ceramics is very important for the optical property improvement.