Abstract: Polypropylene (PP) and polytetrafluoroethylene (PTFE) film were treated by cold oxygen plasma for surface modification. The effects of plasma treatment parameters, chemical structure of materials and gas species on water contact angle (WCA) and peroxide concentration were investigated, respectively. An optimum treatment condition was found as 4 min, 400W and 100SCCM, by which the WCA of PP film decreased from original 99刟 to 7刟. By comparison of the influence of plasma parameters, chemical structure of materials and gas species on WCA and peroxide concentration, the correlation between WCA and peroxide concentration was found, which indicates a correlation between WCA and radical concentration on the film surface generated by plasma treatment.