Effect of sintering atmosphere on the densification and grain growth has been studied in $Al_2O_3$ and $Al_2O_3-MgO$(0.1 wt.%). The compacts were sintered at 1600℃ under various atmosphere for 0.5h to 8h. When the sintering atmosphere was changed from $O_2$ to $N_2$ or Ar after 30min of sintering, higher density and smaller grain size were attained in both $Al_2O_3$ and MgO doped $Al_2O_3$compared with in the specimens sintered in $O_2$. By changing atmosphere from $N_2$ or Ar to $O_2$ higher density and smaller grain size resulted for pure $Al_2O_3$ and, however, lower density and larger grain size for MgO doped $Al_2O_3$. The atmosphere change from $O_2$ to $N_2$ could lead to the highest sintered density and the finest grain size in the systems studied. These effects of sintering atmosphere have been discussed on the basis of the external gas pressure effect, oxygen potential difference in and out of the specimen, and location of the pore site.
