The effect of kind of starting materials used for a sintering additive. magnesium acetate and magnesium nitrate, on the mechanical properties of alumina sintered body made by adding 1000 ppm of the additives, respectively, was investigated. As for the alumina sintered bodies prepared from magnesium acetate and magnesium nitrate, we observed that their relative densities decreased rapidly with increasing sintering temperature 1$600^{circ}C$. Outer layer of alumina bodies had a duplex microstructure consisting of pores and grain growth. Also the inner layer had a second phase between alumina grain boundaries. By EPMA analysis, we confirmed that the grain boundary phase was a compound containing Mg.