Multiprotein bridging factor 1 (MBF1) is known as a transcriptional co-activator that enhances transcription of its target genes by bridging between transcription factors and TATA-box-binding protein in eukaryotes. Arabidopsis thaliana has three MBF1 genes: AtMBF1a-AtMBF1c. However, details of the functions of AtMBF1 remain unclear. For this study, transgenic Arabidopsis overexpressing AtMBF1 fused to an active transcriptional repression domain (SRDX) was constructed. The chimeric protein putatively functions as a transcriptional co-repressor and as a suppressor of functions of endogenous AtMBF1 in transgenic plants. Transgenic Arabidopsis overexpressing AtMBF1-SRDX (AtMBF1-SRDX(OE)) showed an extremely small leaf phenotype under a continuous white light condition. Its leaf cells-especially those around vascular tissues, where strong expression of endogenous AtMBF1s is observed-were much smaller than those from the wild type (WT). In addition, a lower cell number was observed in leaves from AtMBF1-SRDX(OE) plants. Time course analysis of cell size revealed that cell expansion of leaves of AtMBF1-SRDX(OE) plants was dramatically suppressed during the late leaf developmental stage (cell expansion stage), when endogenous AtMBF1b is strongly expressed in the WT. The results show that ploidy levels of leaves from AtMBF1-SRDX(OE) plants were dramatically lower than those from the WT; moreover, expression levels of several negative regulators of endoreduplication were more elevated in AtMBF1s-SRDX(OE) plants than those in the WT. These observations suggest that AtMBF1-SRDX interacts with regulators of endoreduplication. Therefore, AtMBF1s are considered to affect not only leaf cell expansion but also regulation of the ploidy level in leaf cells during the leaf expansion stage.