Abstract: Based on non-equilibrium thermodynamics, the dynamic behaviour of microbial cells trying to achieve maximal growth rate was studied. The growth process was divided into anabolic and catabolic reactions. By optimizing the anabolic flow through the phenomenological relationship, the optimal growth rate and catabolic flow were deduced. Calculations with regard to practical cultivation systems proved that the equations held good for real microbial cells. Futhermore, the metabolic process was divided into three reactions: anabolism , catabolism and the synthesis and utilization of ATP. The same equations as those deduced in black box model were obtained and the relationship of the total coupling degree and the coupling degree of a single step was presented.