Abstract:The meiosis of Ammopiptanthus nanus, an endangered species of Leguminosae, was observed under Olympus Vanox microscope. The materials collected from Wuqia County in Uygur Autonomous Region of Xinjiang were fixed in Carnot's fluid, then washed in absolute ethanol and preserved in 75% ethanol. Slide specimens were prepared by smear method and enzyme-disaggregated method. There are nine bivalents in pollen mother cells at late diplotene or early diakinesis, and the homologous chromosomes essentially arrange on the equatorial plate at late metaphase Ⅰor early anaphase Ⅰ, then the two homologous chromosomes of each bivalent separate from each other and move to two opposite poles drawn by spindle fibers. Nine chromosomes at each pole are observed, therefore the basic chromosome number of this species should be x=9. In addition, abnormal behavior of chromosome is not found in the process of meiosis, so the development of microspores is regarded as normal. It is suggested that the endangerment of the species is not related to the chromosome behavior.