Abstract: The photostabilization mechanism of Tri-(-1, 2, 2, 6, 6-penta-methyl piperidinyl) phosphite (GW-540) for polymer was studied by means of determination of mechanical properties, ESR, IR, oxygen up-take as well as experiment of sensitized photo-oxidation using methylene blue. The experimental results show that the GW-540 possesses a good photostabilizing effectiveness, it can produce stable nitroxy free radical (>NO^·) during the photostabilization process, decompose macro-hydroperoxide and quench singlet oxygen. The protostabilizing action of GW-540 may be due to the auto-synergistic action of polyfuction including hydroperoxide decomposition, free radical seaverging and singlet oxygen quenching.