Abstract: The role of carbon in Mn-Al-C permanent-magnetic alloys was investigated by means of X-ray diffraction and other methods. The phases existing in Mn (72wt%) -Al (28wt%) and Mn-Al-0 (0.5~1.5%) alloys in cast, quenched (from 1150℃) and annealed (450° -650℃) states were determined. A small amount of β phase and Mn₃AlC precipitated in the alloys during high temperature annealing when the carbon content was higher than 1%. The addition of carbon made the formation of the highly-magnetic transtion phase τ easier with better stability, and decreased the eutectoid decomposition temperature up to about 70℃. Addition of carbon in Mn-Al alloy also changed the lattice constants of ε and τ phases. The lattice constant a became smaller and c larger in the tody-centered tetragonal phase τ with increasing amount of carbon, when the carbon content is>0.6%, which is the solubility limit of carbon in Mn-Al alloy. The lattice constant a approached a constant value while c continued to rise, when carbon is >1%, corresponding to the supersa-turation of carbon in Mn-Al alloy. When the supersaturation became too great and the metastable state was destroyed, lattice constant c had a trend to recover and became smaller. The trend of the variation of c/a ratio was the same as that of c. In comparison with the binary Mn-Al alloy, the ternary Mn-Al-lC alloy had a higher degree of long range order in both ε and τ phases. The contribution of carbon to the magnetic property of the alloy is discussed on the basis of the uniaxial strain, the degree of long range order, the dispersed particles of Mn₃AlC and crystal imperfections.