Recent advances in the small angle X-ray scattering technique have made it possible to obtain information on the spatial distribution of protein structure in solution, such as the radius of gyration and shape of the molecule, in a relatively short time. We have studied the various conformational states of horse ferricytochrome c by small angle X-ray scattering. Whereas Guinier plots showed that the radius of gyration of the molten globule state is slightly larger than that of the native state, Kratky plots indicated that their shape is globular. To clarify the mechanism of the formation of the molten globule state, we measured, by the small angle X-ray scattering, the equilibrium transition at pH 2 from the acid-unfolded state to the molten globule state, which was induced either by the addition of NaCl or the acetylation of lysyl amino groups. The Kratky plots at various stages of folding showed an isoscattering point, strongly suggesting a two-state mechanism. For both the NaCl and acetylation-induced transitions, the transition curve determined from Kratky plots agreed well with the transition curves from far-ultraviolet circular dichroism, tryptophan fluorescence, and Soret absorption, further supporting the two-state mechanism.