Abstract Glucose oxidase (GOx) was immobilized on entangled and high surface area carbon nanotubes (CNTs) grown on an alumina substrate, and direct electron transfer reaction between GOx and the electrode was revealed. Fe/MgO catalyst layer was spin-coated on the insulating alumina substrate and the CNT layer was grown on the catalyst by chemical vapor deposition of methane at 950 °C for 15 min. About 20–30 nm bundles of about 1 nm single-wall as well as 10 nm multiwall CNTs are formed. The redox process was surface-controlled and electron transfer coefficient and the rate constant were estimated to be 0.35 and 0.64 s −1, respectively. In addition GOx immobilized on CNT layer showed a linear response range between 12 and 62 μM of glucose concentration. A detection limit and sensitivity of 0.1 μM and 635 μA mM −1 cm −2, respectively, were obtained for the biosensor.