This paper describes the development of a label-free immunosensor based on carbon nanotube (CNT) array electrodes. Highly aligned multi-walled carbon nanotubes were grown on a Fe/Al2O3/SiO2/Si substrate by chemical vapor deposition (CVD). The substrate was patterned with 100 μm square blocks and 100 μm spacing between blocks. Carbon nanotube towers up to 2 mm in height grew from the blocks, and the towers were easy to peel off the silicon substrate. The harvested towers were cast in epoxy and both ends were polished; one end for electrical connection, and the other end for use as an electrode. The nanotube electrode was then electrochemically activated to open the nanotube ends and to expose COOH groups on the surface. Anti-mouse IgG was then covalently immobilized on the nanotube array. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterise the binding of mouse IgG to its specific antibody already immobilized on the nanotube electrode surface. A non-linear calibration plot was constructed based on the change in the electron transfer resistance at the electrode surface as a function of mouse IgG concentrations. A detection limit of 200 ng/mL and a dynamic range up to a 100 μg/mL range were obtained. Overall, it was found that the nanotube array immunosensor was easy to fabricate and has good sensitivity.