Two reversible solid-state phase transitions of erucic acid (cis-13-docosenoic acid), the γ f R and γ1 f R1 phase transitions, have been followed by vibrational spectroscopy (IR and Raman) and X-ray diffraction. The crystal structure analyses of the higher temperature phases, R and R1, have been performed. The symmetry of crystal lattices remains unchanged during the both transitions, i.e., the γ and R phases belong to a monoclinic system (P2 1 /a) and the γ1 and R1 phases belong to a triclinic one (P1 h). At the γ f R transition point, the subcell structure of the methyl-terminal chains is reconstructed, accompanying a large conformational change of cis-olefin groups. On the γ1 f R1 transition, the inclination manner of hydrocarbon chains changes, while there are no significant changes in the subcell structure and the conformation of cis-olefin groups. The conformational disorder at methyl terminals increases continuously for the γ1 f R1 transition, contrary to an abrupt occurrence at the γ f R transition. The unit cell volume also increases gradually in the γ1 and R1 phases and shows no prominent changes at the γ1 f R1 transition, while a stepwise increase takes place at the γ f R transition. The important factors for transition behaviors are discussed on the basis of the crystal structures.