This research is to aim at the study of sintering mechanism through the observation of microstructure by scanning electron microscopy, after the mixture of 30wt% $Al_2O_3$ (White Alundum) powder is fired in air at the temperature range of 1350~150$0^{circ}C$ in order to sinter $Al_2O_3$-Al through the oxidation of Al powder. The results obtained in this experiment are as follows: 1. While the compressive strength of $Al_2O_3$(WA) body fired at $1450^{circ}C$ for 5hrs in air is 150kg/$ extrm{cm}^2$, that of Al-$Al_2O_3$ body fired at 135$0^{circ}C$, $1400^{circ}C$ for 5hrs in air is 1100kg/$ extrm{cm}^2$, 1600kg/$ extrm{cm}^2$ respectively, and the higher the firing temperature, the more the compressive strength increases. These results from the sintering effect between $Al_2O_3$(WA) grains and surrounding Al-oxidation layer. 2. While the compressive strength of Al2O3(WA) body fired at 150$0^{circ}C$ for 5hrs in air is 250kg/$ extrm{cm}^2$, the compressive strength of Al-$Al_2O_3$body fired under the same condition is 2050kg/$ extrm{cm}^2$ and water absorption 9.0%, porosity 23.3%, bulk density 2.60gr/$cm^3$. It is assumed that these results come from not only the grain growth of oxidized Al grains but also the increase of bonding strength between $Al_2O_3$(WA) grains.