Interdiffusion coefficients of manganese D~ were investigated as the functions of solute atom of Mn in FCC-γ phase of Fe-Mn-C ternary alloy at 950℃ and 1000℃ by using the vapour-solid couple method. D~ are gradually increased by the addition of Mn and the values are in the range of D~=0.040-0.240x10^(-10)㎠/s. Diffusion of Mn atoms are accelerated in the matrix which has contained C atoms, but the increment in D~_(Mn) with increasing Mn concentration is lessened in the matrix which has contained C and Mn atoms. In order to study the intrinsic diffusion phenomena, the diffusivity of Mn atoms D_(Mn) were calculated by the equation of Darken and Manning with the thermodynamic factor and structure factor. The difference between D~_(Mn) and D were in the range of 10-40%, and it`s tendency was increased with the concentration of Mn.