The morphology and crystallography of martensite either formed by cooling to subzero temperatures (thermal effect) or by compression deformation were compared for different austenite grain sizes of Fe-29% Ni-2% Mn alloy by transmission electron microscope (TEM). TEM observations revealed both ε and α′ martensite formation within large grained austenite phases by thermal effect whereas only α′ martensite formation was observed in small grained austenite phases. On the other hand, compression deformation effect caused only α′ martensite formation in both small and large grained austenite phases of Fe-29% Ni-2% Mn alloy. Thermally induced α′ martensite exhibited a lenticular morphology with partial twinnings that are peculiar to this kind of morphology. The crystallographic orientation relationship between austenite and thermally induced α′ lenticular martensites was found to be as Kurdjumov-Sachs (K-S) type relationship. © 2006 Elsevier B.V. All rights reserved.