Abstract: Stamens are important reproductive organs which produce pollen in seed plants, and their normal development is related to the reproductive status of plants and affects the yield of crops. Two knockout mutants an3-1 and an3-4 of Arabidopsis thaliana transcription coactivator ANGUSTIFOLIA3(AN3) were identified by RT-PCR. By morphological examination, the stamens of the an3-1 and an3-4 mutants were shorter than those of wild-type, whereas, the pistils were not changed significantly. By constructing AN3 promoter with GUS expression vector, the flower tissues of obtained Pro-AN3-GUS plants were stained and observed. The AN3 promoter in Arabidopsis seed embryos, mature pollen, stigma, petals was expressed. AN3 could be expressed in important tissues such as floral organs during Arabidopsis reproductive growth. These above results are consistent with the short stamens of an3-1 and an3-4 mutants. Thus, AN3 positively regulates the stamen development of A.thaliana.