Abstract: The plant hormone cytokinin plays important roles in cell division and differentiation of floral meristem. To investigate the relation between cytokinin and the flower, as well as floral organ development, we investigated in Arabidopsis the expression of the cytokinin biosynthetic gene IPT4 under control of the promoter of APETALA1 (AP1) in the floral meristem and outer two whorls of floral organs. Transgenic plants with pAP1::IPT4 showed abnormal phenotypes, including acompact flower arrangement and increased number of flower organs. In situ hybridization and GUS staining analyses revealed the up-regulation of LEAFY (LFY) , AP1, PISTILLATA (PI) and AGAMOUS (AG) in pAP1::IPT4 transgenic plants. Thus, the expression of AtIPT4 under control of AP1 mediates the development of flower and floral organs in part via regulating the expression of LFY and floral organ identity genes.