For decades, transformation induced plasticity (TRIP) assisted steels with high tensile strength and exceptional ductility at room temperature have attracted a great deal of attentions. Their applications are often limited due to the low yield strength. In this work, an ultrafine grained (UFG) duplex Mn12Ni2MoTi(Al) TRIP steel with high yield strength and good ductility is fabricated by cold rolling and subsequent annealing at 710~745 ℃. The microstructure and mechanical properties of the steels with different heat treatment conditions are investigated by means of XRD, SEM, TEM, hardness and tensile tests. It is found that after annealing at 710~745 ℃, the deformation microstructure of the cold-rolled samples has transformed into a sub-micron UFG duplex microstructure consisting of austenite, ferrite and dispersed second-phase precipitations. The second-phase precipitations formed during annealing are rich in Ti, Mo and Si, and play an important role in preventing the ultrafine grains from coarsening, which results in high yield strength and good thermal stability. After annealing at 710 ℃ for 24 h, the average grain size of the UFG steel is still less than 500 nm. The elongation of the UFG duplex steel is increasing with the increment of the volume fraction of austenite in the UFG duplex steel at room temperature. The volume fraction of austenite in the UFG duplex steel at room temperature first increases and then decreases with the annealing temperature and time increasing, and reaches the maximum value when annealing at 745 ℃ for 0.5 h. The yield strength and total elongation of the UFG steel can be as large as 900 MPa and 23%, respectively, which are about double those of the quenched martensitic sample.