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 ${gamma}$-and ${ heta}$-phases in the range of temperature from 900 to $1100^{circ}C$, 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 $ar{r}$ and annealing time t is described as follows ; $ar{r}^2-{ar{r}_0}^2=k_0{cdot}t$ 2) The grain growth of duplex, (${gamma}+{ heta}$), strucrure is slower than that single phase because the chemical composition of ${gamma}$-and ${ heta}$-phases differs esch others. 3) The grain of (${gamma}+{ heta}$) duplex structure grow slowly in a mode of Ostwald ripening. Because grain boundaries of ${gamma}$-phase migrate under a restriction of pinning by ${ heta}$-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.