In a few minutes after the electrode assembly was immersed into a buffer solution, (50 mM K2HPO4–KH2PO4 buffer, pH 6.8) the current become constant because the diffusion rate of oxygen into the microbial film from the bulk of the solution reaches equilibrium with the consumption rate of oxygen by endogenous respiration of the immobilized microbes. This current level is named as _initial basal current. When appropriate aliquots from a standard BOD solution were added into the stabilized electrode assembly, the current level decreased as the biodegradable compound diffuse into the microbial film from the bulk of the solution. Then, in few minutes, the current of the dissolved oxygen (DO) probe reached another constant current level known as the ‘final basal current’. The difference between the initial and final basal current values was defined as change in current (∆I). Because the magnitude of the ∆I isproportional to a concentration of immediately biodegradable organic compounds in a sample in a certain range, an unknown BOD concentration in a sample is predictable based on the magnitude of ∆I observed.