The thermal stability of duplex high Mn-steel structure have been investigated using 15%Mn-1.0%∼2.4%C steels which are composed of γ- and θ-phases in the range of temperature from 900 to 1100℃, and time from 50 to 300h. The results are as follows ; 1) The grain growth in single-phase region proceeds by grain boundary migration and the relation between mean radius r-￣ and annealing time t is described as follows ; r￣^(-2)-r￣^(-2)_0=k_0·t 2) The grain growth of duplex, (γ+θ) , strucrure is slower than that single phase because the chemical composition of γ- and θ-phases differs esch others. 3) The grain of (γ+θ) duplex structure grow slowly in a mode of Ostwald ripening. Because grain boundaries of γ-phase migrate under a restriction of pinning by θ-phases. 4) In the duplex structures, the dispersed structures change to the dual-structures, as the volume fraction of the dispersed second-phase increase. Consequently, the growth-law, which is controlled by boundary-diffusion change to that of the volume diffusion-mechanism.