The process of sex differentiation of Acer truncatum, an andromonoecious woody plant, was morphologically and cytologically observed. Results showed that at early phase of sex differentiation, both pistil and stamen primordia could be found in male and bisexual flower buds, but selective abortion occurred in the pistil primordia of male flowers when macrosporocytes developed to megaspore tetrads. Pistils of bisexual flowers could develop and set fruits normally, and the stamens could produce mature two-celled pollen but the pollen could not dehisce. It is suggested that the failure of anthers of bisexual flowers to dehisce might be caused by the low lignification in fibrous layer.