Provided is a method for preparing an oxide based solid electrolyte, the method comprising preparing a mixture including a lithium (Li) compound, a lanthanum (La) compound, and a metal compound, the metal compound including a first metal element represented by M, adding a first precursor including a second metal element and a second precursor including a third metal element to the mixture, and performing a crystallization operation to form a compound represented by LixLa3M2O12 from the mixture having the first precursor and the second precursor mixed therein, wherein the compound is doped with the second and third metal elements.