RUNX1 is a transcription factor that plays critical roles in hematopoietic proliferation and differentiation. Megakaryocyte is a precursor cell of platelets. Several reports have implied that RUNX1 is important in megakaryocytic differentiation and proliferation. However, the mechanism is not well understood. In this study, we employed a megakaryocytic cell line UT-7/GM and suppressed the RUNX1 gene expression by siRNA. Knocking down of RUNX1 induced the increase of megakaryocyte-specific gene expression and down-regulation of polyploidization. RUNX1 overexpression decreased the PF4 and GPIIb promoter activities. These results suggest that RUNX1 promotes proliferation of megakaryocytic cell line but not megakaryocytic gene expression in UT-7/GM cells.