K⁺-β/β" -Al₂O₃ in the temary system K₂O-MgO-Al₂O₃ was directly synthesized by solid state reaction. The phase formation and phase relation were carefully investigated in relation to starting composition, calcining temperature and time, and dispersion medium. The optimal synthetic condition was also examined for the formation of β"-Al₂O₃ phase with a maximum fraction. As a composition range,the mole ratio of K₂O to Al₂O₃ was changed from 1:5 to 1:62 and the amount of MgO used as a stabilizer was varied from 4.2 wt % to 6.3 wt %. The calcining temperature was selected between 1000℃ and 1500℃. At 1000℃, the β/β"-Al₂O₃ phases began to form resulted from the combining of -Al₂O₃ and KAlO₂ and increased with temperature rising. All of α-Al₂O₃ phase disappeared to be homogenized to the β/β"-Al₂O₃ phase at 1200℃. Near the temperature at 1300℃, the fraction of β"-Al₂O₃ phase showed a maximum value with the composition of K_1.67Mg_0.67Al_10.33O₁₇. At temperatures above 1300, the fraction of -Al₂O₃ phase decreased gradually owing to K₂O loss caused by a high potassium vapor pressure, and the appropriate calcining time was about 5 hours. Acetone was more effective than distilled water as a dispersion medium for milling and mixing.