A highly sensitive and selective field-effect transistor biosensor using thermally reduced graphene oxide (TRGO) sheet decorated with gold nanoparticle-antibody conjugates is demonstrated. Probe antibody (anti-Immunoglobulin G) is labeled on the surface of the TRGO sheet through gold nanoparticles and electrical detection of the protein binding (Immunoglobulin G and anti-Immunoglobulin G) is accomplished by FET and dc measurements.