The effects of alloying elements and the conditions of reverse transformation studied treatment on the formation of retained sustenite in 0.15C-6%Mn-(Ti, Nb) steels has been studied. The addition of Ti and Nb to 0.15C-6%Mn steel shows no effect on the formation of retained austenite. In case of reverse transformation treatment at various temperatures, the shape of retained austenite was lath type, growing toward the longitudinal and thickness direction with increasing the heat treatment temperatures. The retained austenite formed by the reverse transformation treatment at higher temperature has a lot of stacking faults induced by the internal stress. The retained austenite was stabilized chemically by enrichment of C and Mn in the vicinity of a untransformed sustenite and the chemical stability of retained austenite was decreased with increasing the heat treatment temperature and the holding time. It was effective to heat treat at 650℃ in order to obtain over 30vo1.% of retained austenite, but more desirable to heat treat at 625℃ for a long time, considering the amount and quality of retained austenite.