At this present work, the process of direct bioelectrocatalytic oxygen reduction by laccase was investigated using electrochemical methods and mathematical modelling. The maximum of achieved current density is 640 μA cm-2. The developed mathematical model includes the mass and potential conservation equations and takes into account the porous structure of the electrode active layer. The equations of the mathematical model were solved by numerical methods with own developed software package. The profiles of the components concentration on time and thickness of the active layer of the electrode were obtained. The adequacy of the mathematical model was tested by comparing the experimental and calculated values of the electrodes activity at different loadings of the carbon material. By the golden ratio method, the optimal electrode carbon material loading of 0.92 mg cm-2 was found.