If there is currency substitution, the central bank will lose independence in monetary policy even if the flexible exchange rate system is adopted. In this paper, we investigate the existence of currency substitution between Taiwan and the United States in an open economy during the period of the managed floating exchange rate system, and examine the role of the factor influencing monetary policy and domestic money demand function derived from a small-country portfolio balance approach. To take account of currency substitution, we use quarterly data over 1981-2005 period on the demand for money and include data on the real exchange rate in addition to real income, domestic nominal interest rate and foreign nominal interest rate. The methodology is based on an application of the Johansen and Juselius¡]1990¡^cointegration technique. Also use error correction model to discuss short-run dynamic adjustment processes of these variables. Application of the Augmented Dickey-Fuller test and Phillips-Perron test indeed reveal that all variables are integrated of order one. The result from the Johansen¡¦s maximum likelihood mehtod reveal that there is only one cointegrating vector among the variables. This implies that there is long-run equilibrium relationship among the variables. There is clear evidence that demand for money is affected not only by changes in domestic variables such as real income, domestic nominal interest rate but also by fluctuations in foreign nominal interest rate and real exchange rate. And the coefficiect of the real exchange rate is negative and statistically significant. That means currency substitution is significant factor in the domestic money demand equation and currency substitution indeed exists in Taiwan. This paper successfully provides a consistent result, currency substitution indeed exists in Taiwan. Therefore, to have an effective monetary policy, the monetary authorities should take into account the international factors.