The purpose of this research is to investigate the lattice constant of κ phase (Fe,Mn)3AlCx at different temperature. The results reveal that the coefficient of thermal expansion (COE) of theκphase Fe3AlCx keeps constant from room temperature to 650℃. On the other hand, the COE of theκphase with manganese content (Fe,Mn)3AlCx exhibits an unusual discontinuous feature at the temperature range of 400~500℃. Moreover the course of heat-treatment plays an important role to govern the lattice constant of the (Fe,Mn)3AlCx specimen. This can be explained by concentration variation of point defects. Meanwhile, the excess increase of the lattice constant shows that the major type of point defect is the anti-structure defect type.