Application of the lattice and structure imaging techniques in electron microscopy has shown that θ-Fe₃C and χ-Fe₅C₂ intergrow microsyntactically in the precipitated particles at the early third stage of martensite tempering (520∼670 K). Also is found the microsyntactic intergrowth of higher carbides, θ_n-Fe_2n+1C_n, having structures closely related with those of θ-Fe₃C and χ-Fe₅C₂. All the forbidden spots observed in electron diffraction patterns from the precipitates can reasonably be interpreted on the basis of the characteristic growth nature of the carbide phases. At the later stage of tempering (720–870 K), the precipitates are θ-Fe₃C containing various kinds of planar defect. A mechanism is presented for the transformation from χ-Fe₅C₂ to θ-Fe₃C.