A mathematical model of amperometric biosensor is analysed. This model is based on catechol-polyphenol oxidase as a prototype electroenzymatic model system. This paper presents an approximate analytical method (He's Homotopy perturbation method) to solve the system of non-linear differential equations for Michaelis-Menten formalism that describe the concentrations of substrates and product within the enzymatic layer. Analytical expressions for substrate concentrations, product concentration and corresponding current response have been derived for all values of parameters using Homotopy perturbation method. These results are compared with simulation results (Scilab program) and are found to be in good agreement. The obtained results are valid for the whole solution domain. (C) 2011 Elsevier Ltd. All rights reserved.