Modifications in the structural and optical properties of 100 MeV Ni7+ ions irradiated cobalt doped ZnO thin films (Zn1-xCoxO, x = 0.05) prepared by sol-gel route were studied. The films irradiated with a fluence of 1 x 10(13) ions/cm(2) were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn-O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV-vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn1-xCoxO thin film due to sp-d interaction between Co2+ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d-d transition of Co2+ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation. (C) 2009 Elsevier B. V. All rights reserved.