This paper investigates whether Tobin tax would be effective to reduce exchange rate volatility. When the rational speculators observe different temporarily shocks and take Tobin tax into account, the exchange rate will have either stable or unstable path through speculators’ changing optimal holdings. If the effect of current account shock dominates the effect of interest differential shock, the imposition of tax will stabilize the currency. This result is consistent with Tobin’s view. On the contrary, if the effect of interest differential shock dominates the effect of current account shock, the imposition of tax will destabilize the currency. The best policy in this case is to let international capitals move freely.