Following Devereux, Shi, and Xu（2007）, this thesis analyzes the effects of tariffs and derives the optimal tariff under the circumstances that the US dollar acts as an international ‘vehicle’ currency— all traded goods prices between the US and the rest of the world are set in US dollars. We set a tariff policy rule which allows the authority to react to country-specific productivity shocks. The analysis on the equilibrium shows that the current tariff imposed by the US on its output is expansionary. However, for the rest of the world, a current tariff imposed by the US is contractionary. Optimal tariff under flexible and fixed exchange rates are examined. Under flexible exchange rate, optimal tariff policy parameters of the US are dependent only on the monetary policy of its own being indifferent to the exchange rate movement；the exchange rate movement is crucial to the optimal tariff policy of the rest of the world. Nevertheless, the optimal tariff policy of the US under fixed exchange rate is the same as that under the flexible exchange rate, while the policy of the rest of the world is altered owing to the loss of control on its domestic money supply to do the unilateral peg.