This thesis looks into the European sovereign debt crisis from a perspective of the Mundell-Fleming trilemma. The European sovereign debt crisis refers to the problems of excessive fiscal deficits and government debt in Greece, Ireland, Portugal, Spain and Italy between 2009 and 2012 which, because of the adoption of common currency, the euro, rendered troubles of individual states to become a crisis of the entire eurozone. Many studies point to weak fiscal discipline as the main cause of the crisis, but a perspective of the Mundell-Fleming trilemma suggests the institutional design of the euro as a more fundamental factor. While one of the purposes for those states to participate in monetary integration was to enable the effective allocation of the capital so as to maximize the overall economic benefits, the achievement of “free flow of the capital” and “exchange rate stability” nevertheless takes as the expense “monetary policy autonomy,” which constrained how states could react when facing fiscal problems. This thesis first reviews the history of the euro, pointing out the background in which the eurozone realized a monetary union but failed to achieve effective fiscal integration. Second, the thesis analyzes the eruption of sovereign debt crisis in each of the five states, indicating how the lack of monetary autonomy prevented them from using depreciation as a policy instrument. The crises of individual member states ultimately called for the intervention of the Troika, i.e., the European Union, International Monetary Fund and the European Central Bank. The thesis in the final part points out that the Troika not only put forth a series of emergency bail-out measures and reform packages for those states in need, but have also come to examine and address issues at the collective and institutional level, with the adoption of the “Fiscal Compact,” among others, as the means to tackle the crisis as well as prevent new ones from happening.