In this research, composite alumina was prepared to add the various promoters by sol-gel method and examined its thermal stability. After sintering at 1,200 ℃, the thermal stability resulted in following order, Si≒La>Ti>Ba≒Ce>Y>Zr≒Mg, in accordance with adding the promoters. Especially in case of silica-added alumina, a phase transformation temperature to α-alumina increased about 150 ℃ and after sintering at 1,200 ℃, it showed to maintain in γ-form and δ-form alumina phase. Also it showed an increase of surface area from 3 m2/g to 71m2/g compared with pure α-alumina. In the case of silica-added alumina, the characterization change of this alumina particle resulted in a delay of phase transformation because Si-O-Al bond was increased when sintered at high temperature. In case of lanthanum-added alumina, there was a sintering delay phenomenon in inter-particles as LaAlO3 structure existed. The existence of lanthanum structure was confirmed by XRD and XPS analysis. It appeared on the alumina surface as La2O3 structure when it was sintered under 1,000 ℃, as the perovskite structure of LaAlO3 at above 1,000 ℃ and as the magneto-plumbite structure of LaAl11O18 at above 1,300℃.