<jats:p>A metastable hexagonal ω-Fe phase with its particle size of about 1-2 nm coexists at the {112}&lt;111&gt;-type twinning boundaries in Fe-C martensite. The ω-Fe lattice parameters are dependent on their corresponding body centered cubic (BCC) phase (aω=2abcc,cω=3/2abcc). The above dependence and the unique distribution of the ω-Fe particles at the BCC {112}&lt;111&gt;-type twinning boundaries inevitably cause the overlapping of electron diffraction spots of fine ω-Fe with those of BCC and its twin during transmission electron microscope (TEM) observations. Thus, in order to understand the nature of the ultrafine ω-Fe phase in carbon steels, we have simulated its electron diffraction patterns by means of the commercial CrystalMaker software. In most of the zone axes, which show the electron diffraction pattern with three sets of diffraction spots [(1) the BCC matrix crystal, (2) the corresponding twin crystal, and (3) the possible twinning double diffraction spots] together, the simulated results have shown that the diffraction spots from the ω-Fe phase are all overlapped with those three sets of spots. However, in some specific zone axes, for example, in BCC [112] directions, the diffraction spots from the ω-Fe phase can be uniquely recognized since the BCC {112}&lt;111&gt;-type twinning boundary plane is perpendicular to the observation direction, and thus, no twinning double diffraction spots should be observed theoretically.</jats:p>