This paper analyzes the conditions under which a financial institution is systemically important. With measuring the level of systemic importance of financial institutions, we find that size is a leading determinant, which confirms the usual “too big to fail” argument. Nevertheless, the relation is non-linear during the recent global financial crisis. Moreover, since 2003, other determinants of systemic importance than size emerge. For example, decisions made by financial institutions on their choice of asset holdings, methods of funding, and sources of income have a significant effect on the level of systemic importance during the global financial crises starting from 2008. These findings help to identify systemically important financial institutions by examining their relevant banking activities and to further design macro-prudential regulation towards reducing the systemic risk in the financial system.