Abstract: Gymnocladus dioicus is a dioecious species with unisexual flowers.Its sex differentiation first undergoes a hermaphroditic stage,then develops to a male or female stage.Two types of flower buds in serial sizes were investigated.Based on the systematic studies on histological anatomy of the sex differentiation in G.dioicus, the authors identified the particular stages and types of the sexual organ differentiation during the sex differentiation.Results showed that the pollen abortion of female flowers occurred at the meiosis stage of microspore caused by the unnormal tetrads with uncomplete formation of cell plate.While the pistil of male flower began to reduce at the macrosporgenesis.Some histochemical techniques were also used to observe the starch grains and proteins accumulated in the process of wall layer ontogeny.The nature of the sex differentiation was discussed finally.