In order to analyze the high temperature phase formation and the sinterability of super ionic conductor K+-β/β"-Al2O3 which is commonly used as a solid oxide electrolyte, the pure K+-β/β"-Al2O3 powder in the ternary system K2O-LiO2-Al2O3 was synthesized by solid state reaction and formed to tube and disk using slip casting method and cold isostatic pressing (CIP), respectively. The slip casting was conducted in an alumina mold with the slurry containing 40 wt% of solid contents and the CIP was carried out under 20 MPa. The samples were sintered at 1600 ℃, 1700 ℃ and 1750 ℃, respectively, and their phase formation and the sintering density were investigated according to the forming method. The samples produced by CIP showed far higher β"-Al2O3 fraction as compared with those by slip casting. On the other hand, the samples by slip casting showed slightly higher sintering density. The relative density reached to about 83% at 1750 ℃ and for 1 h, independent of the forming method. In the case of 90 min socking time, the density was decreased owing to the exaggerated grain growth and the pores by K2O evaporation.