This study examined the phase changes, nitride precipitation and variation in mechanical properties of STS 304, STS 321 and STS 316L austenitic stainless steels after high temperature gas nitriding (HTGN) at temperature ranges from $1050^{circ}C;to;1150^{circ}C$. Fine round type of $Cr_2N$ nitrides were observed in the surface layers of 304 and 316L steels, even more in STS 321. Additionally, square type of TiN was found in STS 321 austenitic matrix too. As a result of many precipitates in the surface layer of the STS 321, it was seen $370{sim}470Hv$ hardness variation depending on the HTGN treatment conditions, and interior region of austenite represented 150Hv. The surface hardness value of STS 304 and STS 316L showed $255{sim}320Hv$, respectively. The nitrogen content was shown 0.27, 1.7 and 0.4% respectively at the surface layers of the STS 304, STS 321 and STS 316L. After the HTGN it was shown the improvement of corrosion resistance of the STS 321 and STS 316L compared with solution annealed steels in the solution of 1N $H_2SO_4$ whereas the STS 304 was not.