Systematical transmission electron microscopy (TEM) studies of the martensitic substructure in quenched Fe-C binary alloys have revealed that the initially formed martensite has twinning structure, and the twin is body-centered cubic {112}< 111 >-type twin regardless of the carbon concentrations. A metastable hexagonal omega-Fe(C) phase with an ultrafine particle-like morphology is distributed at the twin boundary region. In order to explore the common existence of the omega-Fe in nitrogen steels, the martensitic substructures in high-nitrogen martensitic stainless steels at various conditions (as-quenched, subzero-treated and tempered) have been investigated in detail by means of TEM. The omega-Fe with an ultrafine particle size of 1-3 nm has been observed in all the samples. TEM tilting experiment and electron diffraction analysis have revealed that each martensitic lath or plate is composed of {112}< 111 >-type twin structure with the omega particles at the twin boundary region. The martensite morphology and the relationship between the twin and the omega phase particles have been discussed crystal geometrically. The omega particle size variation with subzero and tempering treatment has also been discussed based on a proposed dilation and split mechanism. The existence of large amount of twins up to 550 degrees C simply suggests that the nitrogen atoms have much stronger effect than carbon atoms on the omega phase stability. The present investigation will provide a very clear image about the martensitic substructure in high-nitrogen martensitic stainless steels.