The polymorphic behavior of 1-octadecyl vinyl ether was investigated by DSC and X-ray diffraction measurements under various temperatures. In DSC measurement of 1-octadecyl vinyl ether in the temperature range of -30 to approximately 50 degrees C, four transition peaks were observed on heating, whereas three transition peaks appeared on cooling. The phase-transition behavior was investigated by the repeating scanning DSC measurements. It was concluded that this compound exhibited four crystalline modifications: alpha, sub alpha, beta(0), and beta(1). It was confirmed by the temperature-controlled X-ray diffraction measurement that these phase transitions are attributed to the change of crystal systems from hexagonal packing (alpha form) to a distorted orthorhombic (O perpendicular to') system (beta(1) form) via orthorhombic (O perpendicular to) (sub alpha form) and intermediate beta(0) form, although the beta(0) form has not yet been clarified. In the gamma-ray-irradiation solid-state polymerization for these crystal forms of this compound, the polymerizability of the sub alpha form is higher than that of other forms, and that of the a form is lowest. The polymerizability demonstrated an unusual increase at a temperature of -83.6 degrees C, probably because the cationic polymerization mechanism is dominant over that of the free radical.