Abstract: Chloranthaceae are a family of basal angiosperms with very simple unisexual or bisexual flowers. Hedyosmum orientale Merr. et Chun is a dioecious species with unisexual flowers. The floral morphology and structure of H. orientale conspicuously differ from that of other species and are important in the study of floral origin and phylogeny, especially of the perianth, in angiosperms. We investigated the organogenic processes and anatomical characters related to unisexual flower development in H. orientale Merr. et Chun by scanning electron and light microscopy. Male and female flowers are in the axil of bracts and are usually formed in a thyrse. The staminate flower is subtended by two leaf-like bracts. 150–200 stamens spirally arranged along the spike axis and the phyllome are located at the base of the spike axis. The bract primordium with its axillant flower primordia is dome-shaped in the beginning and then elongates. The bract primordium grows faster than the floral primordium during male flower development. The male floral primordium expands longitudinally and the phyllome primordia are established around the base. Stamen primordia are formed and further develop at the axis from bottom to top. Two female flowers, composed of trifid perianth and inferior ovary, cluster into a cymule with the adjacent floral bracts. The floral primordium grows faster than the bract primordium during the female floral development process. Three perianth primordia are initiated along the sides of the trilateral floral apex. Simultaneously, a concave floral apex appears, which differentiates into an ovary primordium. The floral development of H. orientale Merr. et Chun is unisexual from inception. The floral meristem initiates only androecial or gynoecial organs and does not go through a hermaphroditic stage. Our studies support the view that the flower of H. orientale Merr. et Chun is a primitive character. The trifid perianth of the female flower is homologous to the phyllomes at the bottom of the male flower. They may be the origin of the perianth (sepal and petal).