We have investigated the micro Raman spectra of pristine and 100 MeV swift heavy ions namely oxygen and silver ions, irradiated at fluences of 1 x 10(12) and 1 x 10(13) ions cm(-2) on metal organic chemical vapor deposition (MOCVD) grown GaN epilayers. X-ray diffractometer (XRD) confirmed the wurtzite nature of GaN. Pristine Raman spectra show the good quality of epilayers. On irradiation with light ion, O7+, the order of defects increases and the annealing or ordering of defects occurs in the crystal at higher fluencies. For heavy ion, Ag12+, irradiation results in the increase of disorder for all fluences. A Lorentzian line shape model has been fitted to the Raman spectra of optical phonon mode of E-2(H) (high) and A(1) (LO) peaks. Biaxial stress (sigma), carrier concentration (n) and phonon life-time of first order optical phonon (tau) are deduced from the Raman shift of E-2(H) (high) and A(1) (LO) mode for all the irradiated samples. While with light ion irradiation, sigma deceases where as n and tau increases and for heavy ion irradiation sigma and n increases and tau decreases. (C) 2011 Elsevier B.V. All rights reserved.