This study was carried out on the effect of carbon on the stability of ferromagnetic τ-phase in Mn-Al-C alloys of three different carbon contents (0, 0.18 and 0.27 wt.% at a fixed Mn of 70 wt.%) by means of the isothermal heat-treatment at temperatures ranging 450℃-600℃. The results obtained can be summarized as follows; i. The values of magnetic properties (Br, bHc, (BH)_max) increased with the amount of τphase present, and then these values decreased gradually upon the decomposition of τphase after each of them reached its maximum value. The optimum temperature of isothermal heat-treatment for the highest magnetic properties ranged from 500℃ to 600℃ for varying heat-treating time depending on carbon contents. 2. A T.T.T. diagram calculated from the data for the formation and decomposition of τphase was possible to obtain by means of the quantitative metallography, X-ray diffraction, and the measurement of magnetic properties at each step during the heat-treatment. 3. The higher the carbon content, the more was retarded in the rate of formation and decomposition of τphase. However the effectiveness of this was analyzed to be much great for the decomposition compared with the formation rate of τphase. Consequently, the τphase region in T.T.T. diagram was expanded as the carbon contents increased.