Thin silicon nitride films were prepared on PET by r.f. reactive sputtering. Deposition Rate, reactive mechanisms, the thickness attribution, chemical stoichiometry and impurity were studied by means of RBS, XPS, and ellipsometer. Results show that chemical stoichiometric films with N-to-Si atomic ratio of 4:3 were achieved even at room temperature. Depth profiles of XPS and SIMS reveal that oxide exists only at the interface between nitride and substrate and Ar atoms are buried in the films.