Phase transformation by the Mn-deprivation was observed in the high-Mn twinning-induced plasticity-aided steel. Mn-depletion was induced by the formation of Mn-O oxide during the dew-point control process at temperature above $-20^{circ}C$, which changed austenitic parent phase to multi-grained ferrite. Mixture of Al-O, Al-Mn-Si-O oxides were observed at the grain boundaries of transformed ferrite.