A method for measuring a bonding force between a substrate (50) and a carbon nanotube array (40) formed thereon, wherein the carbon nanotube array includes a plurality of carbon nanotubes. A force gauge (1) including a cantilever (10), a flat-surface probe (20), a movement mechanism (60), and a force sensor (70) is provided. The probe is secured at one end of the cantilever. An adhesive layer (30) is formed on the flat surface of the probe. The probe is moved toward the substrate and is brought into bonding contact with the carbon nanotube array by the movement mechanism. The probe is pulled away from the substrate by the movement mechanism, causing the carbon nanotubes adhered thereto to separate from the substrate. The force at separation is detected by the force sensor, and that force can be converted into an average nanotube/substrate bonding force/strength.