We investigated the effect of HfO 2 film on the sensor response of a carbon nanotube thin film transistor (CNT-TFT) biosensor. Some parts of the CNT in the channel are suspended due to the formation of a network structure. The electrical noise of the CNT-TFT as a transducer is mainly caused by the structural fluctuation of the suspended CNTs in solution. We found that the electrical noise is dramatically reduced by introducing hafnium oxide (HfO 2 ) film onto the CNT-TFT channel surface. This means that the HfO 2 film has a suppression effect of fluctuation on the suspended CNTs and physical adsorption of the impurities such as non-specific molecules. We demonstrate that introducing HfO 2 film is effective for the detection of thrombin molecules using the aptamer-modified CNT-TFT.