A biosensor with improved performance was developed through the immobilization of horseradish peroxidase (HRP) onto electropolymerized polyaniline (PANI) films doped with carbon nanotubes (CNTs). The effects of electropolymerization cycle and CNT concentration on the response of the biosensor toward H2O2 were investigated. It was found that the integration of CNTs into the biosensor system could increase the amount and stability of immobilized enzyme, and greatly enhance the biosensor response. Compared with the biosensor fabricated without CNTs, the proposed biosensor exhibited enhanced stability and approximately eight-fold higher sensitivity. A linear range from 0.2 to 19μM for the detection of H2O2 was observed, with a detection limit of 68 nM at a signal-to-noise ratio of 3 and a response time of less than 5s.