Since the Bretton Wood sistem collapsed in early 1970s, the free floating exchange rate system has been being applied in many countries around the world. The exchange rate system choice is based on some economic advantages that will be obtained from it, considering some economic disadvantages that might be occurred. The valuation toward the advantages and disadvantages of the exchange rate system implementation exactly depends on the national economy it self, because such economy will give some responses in different ways. In a small-opened economy, the effect that occurred in its national economy by the implementation of the floating exchange rate system will be different with the one which is received by a developed country. The exposition about the phenomenon that is happened in the small-opened economy country will become more clearly explainable by Mundell - Fleming - model which is specially developed for those purposes-, and also by some basic theories related with the topic. By using the perfectly capital mobility asumption, Mundell - Fleming model is able to explain the effect of the economic policies implementation, specially fiscal policy and monetary policy, in a small economy country that has already applied the free floating exchange rate system will be different with other countries. Fiscal policy that has been implemented in the country will not significantly alter its national income, but it will cause an alteration in its exchange rate. However, monetary policy implemented in the same countries, will cause changes in its national income as a result of its exchange rate alteration.