The surface modification of polyimide films was carried out by 3MeV Si+ implantation under vacuum at room temperature and ions fluence ranging from 1 × 1012 to 1 × 1015 cm-2. The chemical degradation of function groups and the mechanism of the structure translation in the irradiated layer were investigated by Fourier Transform Infrared （FTIR）

Ultraviolet/Visible （UV/Vis） spectroscopes and electrical behavior measurement. FTIR results show that the absorbance of typical function group decreases exponentially as a function of fluence. The damage cross section of typical bonds of PI was evaluated from the FTIR spectra. Irradiation dramatically enhances the electrical conductivity

and the sheet resistance in our experiment descends nearly 10 orders of magnitude compared with its intrinsic value. The red shifting of the absorption edge from UV to visible reveals the reduction of band gap which results from the increase of the cluster size.