Observations of the morphology and substructure of strain-induced martensites in an Fe-29.6%Ni alloy revealed the formation of butterfly- and lath-like transformation products {11NJ2}(b) [111](b) type transformation twins and a parallel array of straight screw dislocations along [111](b) directions depending upon the amount of plastic deformation. Crystallographic analysis showed the formation of strain-induced martensites with unique {225}(f) habit plane Kurdjumo-Sachs-type orientation relationship relative to the matrix austenite at least up to the large deformation ranges which could easily make the crystallographic measurements unfeasible. It has been shown that the strain-induced martensites are formed with more elongated shapes as the amount of plastic deformation is increased.