An Fe-1.0C-17Cr (wt.%) stainless steel was subjected to subzero treatment to investigated the structure of martensite midrib. The midrib region appeared in the thin plate and lenticular martensites have been observed detailed by TEM. The TEM result reveals that the transformations of thin plate martensite and lenticular martensite are initiated at the same midrib region. The former keeps the lattice-invariant deformation mode of twinning during subsequent growth, whereas the latter combines both twinning and slip modes. Midrib region is a preferential position for carbide precipitations after tempering. M3C and M23C6 carbide were found in this alloy, and related to the ferrite by Bagaryatsky OR and K-S OR, respectively.