A novel electrochemical biosensor for hydrogen peroxide with high electron transfer efficiency was developed using a nanocomplex composed of horseradish peroxidase and Au-nanoparticle. The nanocomplex was prepared through adsorption of horseradish peroxidase on an Au-nanoparticle. The nanocomplexes were easily coagulated and immobilized on an ITO electrode as a monolayer film by simply casting the nanocomplex dispersion and drying below 10 degrees C. The electrode worked as a hydrogen peroxide biosensor featuring high electron transfer efficiency without any soluble mediator at a mild electrode potential of +0.15V vs. Ag/AgCl.