The literature is trying to explain the effectiveness of monetary policy in Indonesia in influencing the exchange rate under fixed and flexible exchange rate regime. The analysis is combined with their capital mobility relationship. The explanation based on the Mundell's famous trinity : The incompatible among fixed exchange rate, independent monetary policy and perfect capital mobility. The object of the research is Indonesia exchange rate experience between 1970-1994. Under the condition of sticky domestic price level and uncovered purchasing power parity, by using the two-countries asset-market model, the estimation of Indonesia exchange rate movement under fixed exchange rate regime (1971.1-1978.3) proved the Mundell's theorem, that the monetary policy does not compatible with the fixed exchange rate, while the liberalization of capital account did not followed by the opening of capital market, which made the condition of imperfect capital mobility because the capital is easy to move out but hard to move in. The condition was exacerbated with the financial repression at the time. Meanwhile under the preliminary period of flexible exchange rate, besides the imperfect capital mobility condition issue whis is caused by the not-well developing capital market, there is also a sequencing issue which impede the monetary policy. The issue was due to the lateness of the government to move the financial barriers under financial repression : The peg of interest rate and the peg of credit ceiling. While the flexible exchange rate regime had begun on the late of 1978, the financial repression barriers just removed out in the mid of 1983. One compulsory macro-economic condition which has to be fulfilled in order the monetary policy to have effectiveness in influencing the exchange rate under the flexible regime is the ability of interest rate to move up and down as needed. Otherwise, the result is somewhat alike with the monetary policy under the fixed exchange rate. This is shown up by the estimation in the period 1978.3-1983.2. This also explain why the 1982 monetary policy did not have the influence on the nominal exchange rate fluctuation, but succesfully made the inflation move slowlier. The estimation under flexible exchange rate after 1983 shows that the monetary policy is compatible with the flexible exchange rate, but still before 1988, the capital market had not been developed. The capital market was develop after the 1988 deregulation policy and it was integrated with the international market after 1992. This explain that some of the monetary policy had have the influence on the small fluctuation of the nominal exchange rate. Among those are the 1987 sterilitation policy and the 1990 tight money policy. While the 1991 sterilization policy, did not create any fluctuation on the nominal exchange rate. A small apreciation at the end of 1993 1 believed, was caused by the movement of short run capital. The estimation for the causality of the monetary policy and capital mobility to the exchange rate has shown that for the observation period, the monetary policies which have the effect on the variance of the exchange rate are the removed of the peg of the interest rate out, the sterilization policy and the 1988 reserve requirement policy. Capital mobility did not have the effect to the exchange rate variance.