The definition of proportionality of the mathematical model is provided based on proportional relation and value rules, which sets the quantitative criteria to evaluate the quality of form design. The best proportionality is summarized for the form design of numerically- controlled machine tool by the analysis of proportionality between the whole and the key-parts, and a method of optimal design is carried on according to the computation of proportionality value within the key-parts. The conclusion is that the quantitative principle of proportion esthetics and the proportion analysis of modeling of numerically-controlled machine tool will theoretically and practically play an essential role on computer-aided design.