Alumina (Al2O3) is mainly used as a structural ceramic material and to have good mechanical properties requiresa dense microstructure. In commercial fabrication, the liquid phase sintering process is adjusted to reduce the sinteringtemperature of alumina. In this study, the effect of added amounts of cobalt oxide as a coloring agent on the microstructureand mechanical properties was investigated in the CaO-SiO2-MgO-system liquid phase sintering of 92 % alumina at varioussintering temperatures. When 11 wt% Co2O3 was added, a rearrangement of alumina particles, which is the main densificationstep in liquid phase sintering, occurred from a sintering temperature of 1200oC. Solution re-precipitation and coalescencesteps followed from 1300oC with the grain growth of alumina particles. The addition of excess Co2O3 and sinteringtemperatures above 1400oC resulted in a decrease in sintered density and Vickers hardness, because of the low viscosity ofthe liquid phase. In 92 % alumina with the addition of 11 wt% Co2O3, a sintered density and Vickers hardness of 3.86 g/cm3and 12.32 GPa, respectively, were obtained at a sintering temperature of 1350oC.