The microstructures of aluminas, included of dissolved CaO as $alpha$-alumina seeded pseudo-boehmite hydrosol was gelled in plaster mold and doped of MgO as dipping of calcines(120$0^{circ}C$-2h) into Mg-nitrate solution, were compared to the one of which additives are excluded during the gellation. It was formed the boundary layer of 300~350${mu}{ extrm}{m}$ distance from surface to the inside, containing of approximately 500ppm CaO by dissolved Ca from plaster mold. As the MgO addition to the boundary layer with dissolved CaO, the microstructure of the layer was uniformed and inhibited the grian growth, compared to one of that additives be excluded specimen and of MgO doped-inside region. This result was considered as abnormal grain growth and effect of flat boundary formation be appeared by effects of dissolved CaO, were decreased by MgO co-doping.