Piezoresistive effect of carbon nanotube films was investigated by a three-point bending test. Carbon nanotubes were synthesized by hot filament chemical vapor deposition. The experimental results showed that the carbon nanotubes have a striking piezoresistive effect. The relative resistance was changed from 0 to 10.5×10~(-2) and 3.25×10~(-2 ) for doped and undoped films respectively at room temperature when the microstrain under stress from 0 to 500. The gauge factors for doped and undoped carbon nanotube films under 500 microstrain were about 220 and 67 at room temperature, respectively, exceeding that of polycrystalline silicon (30) at 35℃. The origin of the resistance changes in the films may be attributed to a strain-induced change in the band gap for the doped tubes and the defects for the undoped tubes. Piezoresistive effect of carbon nanotube films was investigated by a three-point bending test. Carbon nanotubes have a striking piezoresistive effect. The experimental results showed that the carbon nanotubes have a striking piezoresistive effect. The relative resistance was changed from 0 to 10.5 × 10 ~ (-2) and 3.25 × 10 ~ (-2) for doped and undoped films respectively at room temperature when the microstrain under stress from 0 to 500. The gauge factors for doped and undoped carbon nanotube films under 500 microstrain were about 220 and 67 at room temperature, respectively, exceeding that of polycrystalline silicon (30) at 35 ° C. The origin of the resistance changes in the films may be attributed to a strain-induced change in the band gap for the doped tubes and the defects for the undoped tubes.