The nitrided layer formed on Type 304 steel after gas nitriding was studied using TEM and so on. (1) Dependency of the nitriding temperature and time on the depth of the nitrided layer was studied. The following equation was obtained. L2=8.7*108*EXP(-146,000/RT)*t (2) Nitrided layer formed at both 570°C for 3 hrs and 410°C for 48 hrs had high density of dislocation, stacking fault and lattice strain. (3) Nitrided layer formed at 570°C was mainly composed of є-Fe3N, and had much Fe4N and CrN. (4) On the other hand, the nitrided layer formed at 410°C was mainly composed of S-phase, є-Fe3N was not detected and Fe4N and CrN were very little. N content in the nitrided layer formed at 410°C was about 7-8 mass%. (5) Nitrided layer formed at 410°C showed good corrosion resistance.