The thermal polymerization of aziridine monomers without a mesophase was performed in rubbing and non-treated sandwich cells. Cross-linked polymers, having homogeneous and homeotropic structures, were prepared. The thermal polymerization process was examined by X-ray diffraction measurements and polarizing microscopy. The cross-linked polymers exhibited a smectic A layered structure. In the rubbing sandwich cell, an aziridine monomer with an ethanol terminal group gave a cross-linked polymer having a homogeneous alignment. The homeotropic structure was obtained by thermal polymerization of an aziridine monomer with a nitro terminal group in a non-treated sandwich cell. The homeotropic structure was also formed on the surface of a glass plate and an aluminum thin film. A uniaxial-optical property of the homeotropic structure was characterized by conoscopic observation. The X-ray diffraction measurements showed the formation of the smectic A domain within an isotropic fluid by thermal polymerization at 100°C as well as optical texture observations. The X-ray diffraction pattern of the homeotropic structure was composed of first, second, and third inner reflections and a wide-angle halo, corresponding to the smectic A fluid phase.