Japonolirion osense, the sole species of the genus, endemic to Japan, which is placed together with Petrosavia in the Petrosaviaceae and the order Petrosaviales, is still poorly known with respect to systematic characters. Here I present an embryological study of the anther, ovule, and seed of J. osense. Japonolirion is characterized by a glandular anther tapetum, simultaneous cytokinesis in the microspore mother cell, two-celled mature pollen grains, anatropous and crassinucellate ovules, a two-cell-layered nucellar cap formed early in ovule development, antipodal cells hypertrophied in post-fertilization stages, the ab initio cellular mode of endosperm formation, and exotegmic seeds. Comparisons with the basal monocots Acorus (Acorales) and Araceae (Alismatales), and with the more derived monocots Nartheciaceae (Dioscoreales) and Velloziaceae/Triuridaceae (Pandanales), showed that Japonolirion is clearly distinct from those basal and more derived monocots, supporting a distinct position for Petrosaviaceae or Petrosaviales within the monocots. Extensive comparisons further suggest that the two-cell-layered nucellar cap, whose cells are rich in cytoplasm at the time of fertilization in Japonolirion and thus obviously function as the obturator, is likely to be a common characteristic of the basal monocots and may even be a link with the magnoliids.