Conductive polymer based sensors are potentially a powerful tool for detecting a variety of analytes in gas phase. However, the accuracy and reliability of such sensors are threatened by humidity interference, which limits their use in demanding applications, e.g., air-quality monitoring and health care. Thus, there is an urgent need to suppress the response of conductive polymers to humidity. In this work, the surface of polyaniline (PANI) was modified by fatty acid to block the proton exchange between water molecules and PANI polymer chains and thus suppress the humidity response toward PANI based gas sensors. The results show that the humidity response suppression effect is depending on the chain length of surface modified alkyl acid. The long chain alkyl acid modified PANI sensors exhibit strong suppression of the humidity sensing response; while the short chain modified PANI sensors have weak suppression of the humidity sensing response. In particular, the Behenic acid modified PANI sensors have a concentration depend water vapor response as low as 0.008 %/ppm, which is only 1/5 of the unmodified PANI sensors. This work might be a valuable example of the humid interferon suppression for other chemical sensors.