Polycrystalline Cabeta"/beta type-Al2O3 made by direct sintering has been studied by selected area electron diffraction (SAED), EDX and high-resolution electron microscopy (HREM). There is a new phase besides Cabeta"-Al2O3 in the electrolyte. This new phase is different from Cabeta"-Al2O3 and is called modified Cabeta"-Al2O3 (M-beta" phase). It has a rhombohedral structure with lattice parameters a = 5.63 angstrom, c = 81.51 angstrom. Its composition is nearly Al : Ca = 14:1. The ionic conductivity which is very high is also discussed.