The microstructural changes of Fe-20Mn-12Cr-1Cu alloy have been studied during high temperature gas nitriding (HTGN) at the range of $1000^{circ}C{sim}1150^{circ}C$ in an atmosphere of nitrogen gas. The mixed microstructure of austenite and ${varepsilon}$-martensite of as-received alloy was changed to austenite single phase after HTGN treatment at the nitrogen-permeated surface layer, however the interior region that was not affected nitrogen permeation remained the structure of austenite and ${varepsilon}$-martensite. With raising the HTGN treatment temperature, the concentration and permeation depth of nitrogen, which is known as the austenite stabilizing element, were increased. Accordingly, the depth of austenite single phase region was increased. The outmost surface of HTGN treated alloy at $1000^{circ}C$ appeared Cr nitride. And this was in good agreement with the thermodynamically calculated phase diagram. The grain growth was delayed after HTGN treatment temperature ranges of $1000^{circ}C{sim}1100^{circ}C$ due to the grain boundary precipitates. For the HTGN treatment temperature of $1150^{circ}C$, the fine grain region was shown at the near surface due to the grain boundary precipitates, however, owing to the depletion of grain boundary precipitates, coarse grain was appeared at the depth far from the surface. This depletion may come from the strong affinity between nitrogen and substitutional element of Al and Ti leading the diffusion of these elements from interior to surface. Because of the nitrogen dissolution at the nitrogen-permeated surface layer by HTGN treatment, the surface hardness was increased above 150 Hv compared to the interior region that was consisted with the mixed microstructure of austenite and ${varepsilon}$-martensite.