In order to scrutinize the floral development of the unisexual Betula alnoides and to discuss the organogenesis throughout other members of Betulaceae, the inflorescences, cymules, and flowers of B. alnoides were examined using scanning electron microscopy. In staminate flowers, there is no vestige of the carpel, and a 90 degrees change in the orientation of two stamen primordia was noted during the developmental process. Tepals are usually laminar but vary in number and morphology. Secondary bracts are laminar, similar to the tepals. Pistillate flowers lack stamen primordia; tepals are arrested in early development, and the secondary bracts are more reduced than bracts in the male flowers. Despite this contrasting morphology, the organogenesis pattern of male and female cymules and flowers is similar. Bracts and tepals keep the same arrangements in both genders. The orientation of stamens and carpels is also consistent. Phylogenetic relationships within the family are discussed. The division of Betulaceae into Coryleae and Betuleae and the sister group relationship between Betula and Alnus are reinforced in the aspect of floral ontogeny.