A transmission electron microscopy and diffraction study of butterfly martensite in Fe-Ni-Cr-C alloys was carried out with particular attention to details of the martensite substructure. It was confirmed that a butterfly martensite is consisted with two plates with (252)f and (252)f habit plane variants having the junction plane near {112Ib twins, lengthy dislocations and {lO1>b planar defects were observed as substructures of butterfly martensite. Observed {112Ib twins were not shaped as parallelograms, and twin edges Anside a martensite plate were not parallel to the twinning shear direction [111]b in contrast to those in Fe-Ni lenticular m art en site. The majority of the lengthy dislocations were seen parallel to which is close to the shape strain direction of the (252)f variant. (O1l)b planar defects considered to be produced by the coalescence of martensite platelets were often observed. In the surrounding austenite slip on planes probably produced by the accommodation of the transformation shape strain were observed. In general, although butterfly martensite exhibits a unique paired morphology, its substructures were found to be quite similar to those observed in other morphology of martensite with the same {225lf habit.