Stephania is the sole genus in the basal eudicot family Menispermaceae that possesses both actinomorphic and zygomorphic flowers. Variation in perianth merism can have an important impact on flower symmetry and thus eminent biological significance in evolution of Menispermaceae. Using SEM, we studied the floral development of four representative species, which present the two predominant floral patterns of the genus, namely, homomorphy of both male and female flowers (actinomorphy) or heteromorphy(actinomorphy/zygomorphy). The sepals of the male flowers are arranged mostly in two alternate whorls of three or four each, whereas in female flowers they are in a single whorl of three or four or there is only a single sepal. Petals of male flowers are in a whorl of three or four organs, whereas female flowers of some species have only two petals. Trimerous and tetramerous perianths can coexist in the same umbellets of some species. Variation in perianth merism and loss of perianth parts of the female flowers may result in flower symmetry switching from actinomorphy to zygomorphy. The two main floral patterns are consistent with a distinction of two subclades within Stephania. The unicarpellate genera of the Menispermaceae share a unique combination of characters, including a synandrium, unitegmic ovules, and absence of vestigial sexual organs. However, Stephania differs from its unicarpellate relatives by two features: a two-whorled arrangement of floral organs of the male flowers and a free perianth. The investigation provides new and valuable developmental information on flowers of the little-known Menispermaceae and provides a background for a discussion of the evolution of merism and unisexual flowers in the basal eudicots.