In an effort to elucidate biological functions of transcription factors of Brassica rapa L. (ssp. pekinensis), an NGATHA homolog, BrNGA1, that belongs to the B3-type transcription factor superfamily was identified and expressed in Arabidopsis thaliana under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Arabidopsis plants overexpressing BrNGA1, named BrNGA1ox, displayed markedly reduced organ growth compared with the wild type: lateral organs, such as leaves, flowers and cotyledons, were small and distinctively narrow, and their root growth was also severely retarded. Reduced sizes of BrNGA1ox organs were mainly due to reduction in cell numbers. Kinematic analysis of leaf growth revealed that both the rate and duration of cell proliferation declined during organogenesis, which was consistent with the reduced expression of cyclin genes. Reduction in organ growth was strongly correlated with the small size of meristematic cell pools in the shoot and root meristems. Taken together, these data indicate that BrNGA1 acts as a negative regulator of cell proliferation and may do so, in part, by regulating the size of the meristematic cell pool.