|High Temperature Gas Nitriding(HTGN) of STS 347 and STS 310S austenitic stainless steels has been investigated in an atmosphere of nitrogen gas at the temperature range between 1050 ℃ and 1150℃. The phase change, the hardness variation, precipitates, the nitrogen content and the corrosion resistance of the HTGN treated steels were examined. The surface layer of 347 austenitic stainless steel appeared the nitrides of rod type and round type, and fine precipitates were observed in the austenite, while the surface layer of STS 310S autenitic stainless steel appeared nitrogen pearlite, which was composed of Cr2N and austenite alternatively, at the outermost surface layer. The surface hardness of STS 347 showed 250~370Hv, depending on the HTGN conditions, while the interior austenite phase represented ~175Hv. The surface hardness of STS 310S showed 270~410Hv, while the interior austenite phase region represented 160Hv. The nitrogen content was analyzed 1.4% and 1.6%, respectively, at the surface layer of STS 347 and STS 310S steels. For both STS 347 and 310S austenitic stainless steel, the corrosion resistance of nitrogen permeated steels were superior to that of solution annealed steels.