It is different from traditional considerations that the concentrations of cell growth (X), substrate consumption (S), and product formation (P) during a fermentation process are put into a three-dimensional space (X, P, S). It is found that the coordinate representations for cell growth, substrate consumption, and product formation at different times are maintained on a plane basically. It is called the fermentation plane. Its equation is : 1/YGX + 1/YPP + S = K. The applicability of the new concept and its plane model is demonstrated with six fermentation processes. Satisfactory results are obtained in all cases.