The biocatalyst electrodes, that is, the oxidoreductase-immobilized electrodes in which the electrode behaves as a substitute for a chemical electron acceptor or donor of oxidoreductase reaction could be used in such novel applications as biosensors, bioreactors and biofuel cells. The presence of an electron-transfer mediator between the electrode and the enzyme is useful to accelerate the electrocatalysis at the electrode. Mathematical analysis of the diffusion coupled with the enzyme reaction of both substrate and mediators in the immobilized enzyme layer on the electrode surface has revealed important parameters for understanding the process at the biocatalyst electrode with electron-transfer mediator. The apparent enzyme rate constant and Michaelis constants to characterize the bioelectrocatalysis at the electrode was defined and discussed. Also, the collection factor, that is, the ratio of the amount of mediator collected by the electrode to the total amount of mediator produced in the enzyme layer was defined and discussed. The current-potential curve was derived. A film-coated glucose oxidase (GOD)-immobilized gold minigrid electrode with p-bezoquinone-mixed paste and a film-coated GOD-immobilized platinum mesh electrode with oxygen permeable membrane were designed. These GOD electrodes worked satisfactorily as biocatalyst electrode to oxidize D-glucose electrocatalytically. Characteristics of the GOD electrode with mediator for electrochemical conversion of D-glucose to D-gluconate were studied in terms of the current density and the electrode potential of the electrode. Use of biocatalyst electrodes in organic syntheses was discussed.