Processes of the carbide formation upon low temperature tempering in Fe-4.1 mass%Mn-1.25 mass%C and Fe-5.2 mass%Mn-0.93 mass%C alloys have been examined by means of X-ray and electron diffraction and electron microscopy. Upon the tempering at 358 K, the martensite lattice parameters change in two stages (the preliminary and the first stages according to the former definition) as a function of the tempering period of time. In the preliminary stage, a long period ordered tetragonal phase, being a modification of Fe₄C carbide, is formed as found by Nagakura et al. in Fe–C alloys. Another diffraction effect suggesting the formation of a modulated structure and/or a short range order of carbon atoms has also been observed. The so-called intermediate carbide in the first stage is identical to “η-carbide” found by Hirotsu et al. in Fe–C alloys, exhibiting an orthorhombit structure and superlattice reflections due to carbon atom ordering in diffraction patterns. The lattice parameters of the η-carbide are a=0.470 nm, b=0.429 nm and c=0.285 nm, and the orientation relationship to the martensite matrix is (011)_M//(010)_η, [100]_M//[001]_η. The shape of carbide is a very fine disk or ribbon whose largest areal plane is parallel to the {100}_M or {011}_M plane, and the carbides parallel to the {100}_M and {011}_M planes are arranged along the ⟨100⟩_M and [001]_M directions, respectively, forming an apparent rod or plate shape parallel to the {100}_M or (001)_M plane.