The effects of high austenitizing temperature and the deformation of austenite matrix below the range of strain-induced martensite formation on the morphology, substructure and crystallography of martensite formed in different Fe-Ni-C alloys have been studied by means of transmission electron microscopy. The formation behaviours of both thermal and stress-assisted martensites were examined under various physical conditions and martensite morphology was found to be closely dependent on the high austenitizing temperatures besides the influence of austenite deformation. Although the orientation relationship between austenite and thermally induced martensite was found as the Kurdjumov-Sachs type, it was also observed to change to Nishiyama-Wasserman type in the samples transformed under the stress-assisted conditions.