The phase relationships between the ferrite phase (α) and the austenite phase (γ) were investigated in the iron-rich comer of the Fe-Cr-Mn-Ni quaternary system at 1000°C. The phase boundary between the α and γ phases was determined using a diffusion-multiple technique. In this technique, three end members, FeCr, FeCrMn and FeCrNi alloys, were coupled inside a metal tube, and then heat treated at 1300°C for 24h to create a microstructural state with a composition gradient of Cr, Ni and Mn within the composition range of the end members. This was followed by long time heat treatment at 1000°C for a period of 240 h to attain equilibrium states between the γ and α phase in the composition range. The phase boundary between the γ single phase field and the γ+α two-phase field was found to slightly shift in a way to expand the γ single phase field when the manganese content was increased from 0 to 5 wt%. The shift of the phase boundary was not obvious at chromium contents above 20 wt% and nickel contents above 8 wt%. These results indicate that manganese is a weak austenitizer at low chromium and nickel contents.