In the present work we have reported the effect of Shift heavy ion (SHI) irradiation on the gas sensing properties of tantalum (Ta)/Polyaniline (PANI) composite thin film based chemiresistor type gas sensor for hydrogen gas sensing application. PANI was synthesized chemically by in situ oxidative polymerization method. The thin sensing films of PANI were deposited onto finger type Cu-interdigited electrodes using spin cast technique and a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chemiresistor sensor. These chemiresistor sensing films were irradiated with energetic Au+12 ions (150 MeV) at the different fluencies ranging from 1 x 10(9) to 1 x 10(11) ions/cm(2). The structural and morphological properties of these composite thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements before and after SHI irradiation. The electrical properties of these composite thin films were characterized by I-V characteristic measurements. The changes in resistance of the composite thin film sensor were utilized for detection of hydrogen gas. It was observed that after SHI irradiation Ta/PANI composite sensor shows a high response value and sensitivity with good repeatability in comparison to the pristine sample. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.