Geographic Zone Differentiation (GZD) is one of the basic principles of geography research. Grey cluster method is useful and effective in research of classification. But if the Method of Grey Incidence Cluster (MGIC) is applied to research of GZD directly, the result is not completely correct and is not in accordance with the fact. The reason is that the definition of Absolute Degree of Incidence (ADI) is not suitable to GZD. When it is defined as follows, the conclusion drawn by applying MGIC to GZD is correct and satisfying. In these formulas, ε_ij is the ADI between sample i and sample j; x_i(k) is the value of the index k of sample i. How to choose and decide the indexes of GZD is also brought out, whose principles are: 1) the index is dominant and effective; 2) the values of the index in varied samples are disparate; 3) the indexes are independent and can't be relevant in linearity; 4) the indexes are distinctly different in reflecting geographic sight.