The effect of heterojunctions between polyaniline (PANi) and nitrogen-containing carbonfibers (NCFs)on NO gas-sensing properties were investigated, and the influence of the heterojunction area on the NOgas-sensing performance was confirmed. We prepared NCFs and nitrogen-containing activated carbonfibers (NACFs) from polyacrylonitrilefibers and then loaded them with PANi so they could be used asgas-sensing materials. In the NO gas-sensing tests, the NCFs and pristine PANi exhibited p-typebehavior with decreased responses over time, and the PANi-loaded NCFs (PANi/NCFs) exhibiteddifferent behavior with a very high response (281.0%). This high response can be attributed to theformation of p-p heterojunctions at the interfaces between the PANi and NCFs caused by the graphiticnitrogen in the NCFs. Moreover, the response of the PANi-loaded NACFs (PANi/NACFs) was 2.2 timesthat of the PANi/NCFs, which is due to the activation of NCFs increasing the p-p heterojunction area ofthe PANi/NACFs.