Abstract: The light protection behaviour of hindered amine light stabilizer Tinuvin-770 is studied by IB, BSR, NMR as well as observation of a mechanical property of Tinuvin-770 containing polypropylene film.The results indicate that this light stabilizer shows an optimun concentration (0.3%, W/W) in photooxidation protection of polypropylene.With the concentration of Tinuvin-770 higher or lower than the optimum, the light protection property markedly deteriorates.This phenomenon may be related to the duality of sensitizing reaction and stabilization action of Tinuvin-770.The experimental results indicate that Tinuvin-770 is easy to photolyse under the experimental conditions reported.The free radicals produced by photolysis become the origin of photooxidation initiation.Careful ESR observation shows that the light stabilization action of Tinuvin-770 is due to the formation of the stable free radical