Polyethylene terephthalate (PET) films in thickness of 12 $\mu$m are irradiated by Xe and Au ions at the energies of 9.5 and 11.4 MeV/u and with the ion fluence from $5\times10^{9}$ cm$^{-2}$ to $1\times10^{11}$ cm$^{-2}$. After irradiation, ultra-violet lights are used to illuminate the samples with latent tracks at the wavelength of 365 nm with flux density of 4.2 mW/cm$^{-2}$. UV-irradiation effects on tracked PET are investigated by the UV-vis spectrum and positron annihilation lifetime spectroscopy (PALS). It is found that carbonaceous clusters in PET films are generated by ion irradiation and decomposed with UV illumination by calculating the optical energy band gap $E_{\rm g}$ in the UV-vis spectrum. The free volumes behave differently in track and bulk after UV illumination. In our experiment, the PALS results show an increase in radius and density of free volume in tracked PET films after UV treatment, which indicates an expansion in radius of latent tracks.