The study is to explore the formation .of tertium comparationis in comparative education for manifesting the core elements of comparative education, which is addressed as ”comparability”. First of all, the formation of tertium comparationis is based on the construction of the CIPP model and is intended to present the dynamic process of constant adjustments. The researcher should collect bountiful data in advance, analyze the historical context factors from different comparative units, integrate opinions of different scholars and experts, confirm the category of comparison, engage in the analysis of variables, form systematic comparisons, and pay close attention to ”others' ” voice. Accordingly, the researcher can form the approximate comparison criteria and the criteria of differences to facilitate the preliminary comparison. However, reflexive thinking should be taken into account in process of comparing for the formation of tertium comparationis in comparative education.