The spontaneous martensitic transformations were investigated by using the Fe-base alloy containing 17 per cent Cr and 8 per cent Ni. The isothermal martensitic trans-formations occurred while it was held at above the M_s^ temperature and the T.T.T. diagram was of the form with two noses at about -100°and -135℃, respectively. According to X-ray analysis h.c.p. structure (ε) was formed at the upper nose. The driving force (ΔF^) of transformation in this alloy was 22 cal/mol. This value was derived from an assumption in which the transformation would occur owing to a very low stacking fault energy. Furthermore, from this assumption, it was suggested that the γ→ε transformation would occur at above 7 per cent Ni content. This suggestion agreed closely with practical data. The b.c.c. martensite (α\u27) grew when it was held longer in the temperature range of the upper nose. The habit plane of this α\u27 was (111)_γ plane, whereas the habit plane of the α\u27 formed isothermally in the temperature range of the lower nose and athermally at a temperature below M_s^ was (225)_γ plane
