In this study, the oxidation reduction potential (ORP) of biological denitrification processes is interpreted based on the MIRROR model No. 1, a linear non-equilibrium thermodynamic model developed in an earlier study. The model interconnects the affinities of catabolism and anabolism, the driving forces of microbial metabolism, with the system ORP and reaction rates of biological processes. Experimental results reported in the literature were used for calibrating the MIRROR model No. 1 to determine the optimal values of model stoichiometric, kinetic, and phenomenological parameters; the calibrated model was then used to simulate laboratory data. The simulation results agree well with the experimental observations. There is a close relationship between the affinities of catabolism and the system ORP of the biological denitrification process, but the ORP variation per unit affinity of catabolism is not a constant but proportional to the molarity of electrons transferred catabolically. The linear relationship between redox potential and reaction rate, which is derived based on MIRROR model No. 1, is subsequently verified by the experimental results reported in the literature. This linear relationship enables evaluation of the biological denitrification rate based on the real-time monitoring of the system ORP.