Objective:To study the relationship between plasma treatment time and efficacy. Methods:Test specimens were prepared from an acrylic resin denture material in the size of 2 mm × 10 mm × 10 mm. Plasma treatment was carried out on the surface of Polymethyl methacrylate(PMMA) at different time. XPS studies, IR spectra studies and measurement of wetting angle were performed. Results: XPS showed the peak corresponding to C-O getting higher as the treatment proceeded, however at 120 seconds, the peak did not increase any longer and partly crossed with the peak at the duration of 60 seconds. IR spectra showed the wave corresponding to C-H was reduced as O2-plasma treatment proceeded, and then changed little. Wetting angle initially decreased dramatically, however, as the reaction proceeded, wetting angle increased slightly. Conclusion:Equilibrium was reached for introducing oxygen-containing groups and changing of C-H. As the treatment proceeded, wetting angle increased slightly.