The precipitation behavior in Fe-Mn-Cu austenitic alloys was studied by means of the hardness test, electric resistivity measurement, Mossbauer effect measurement, and optical and transmission electron microscopy. The main results obtained are as follows: In Fe-Mn-3Cu, Fe-Mn-5Cu and Fe-Mn-7Cu alloys, an age-hardening phenomenon was observed and the degree of hardening, became remarkable with increase in Cu content. In this alloy system, a two-phase decomposition was expected to occur and it was considered to be based on a miscibility gap in the Fe0.5 Mn0.5-Cu quasi-binary system. Namely, it was suggested that these Fe-Mn-Cu alloys were decomposed into a Fe-Mn rich (Cu-poor) phase and a Cu-rich phase. By means of splat-quenching from the liquid state an austenitic single phase of Fe-Mn-40Cu alloy was obtained and the two-phase decomposition was also expected to occur in this alloy.