Samples of the AISI 304 austenitic stainless steel nitrided at low temperature and low pressure by low pressure plasma are source ion nitriding have been examined by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy and microhardness tester. A high nitrogen content layer with a thickness of approximately 10 mum which is very hard and f.c.c. phase was obtained by nitriding at similar to 420 degreesC for 70 min at pressure of 0.4 Pa. The results of XPS suggest that the nitrogen induced by nitriding process be in the state of solid solution. The transmission electron microscopy results are consistent with that of previous X-ray diffraction observations.