當上游廠商採取差別訂價，且兩市場規模差異不大時，若貿易自由化使得廠商A跨國銷售，進口國的消費者剩餘反而可能因廠商A的加入而減少，進口國廠商的利潤則因為廠商A的加入而一定減少，因此，自給自足下進口國的社會福利，可能高於貿易自由化使廠商A進入B國銷售下的社會福利。 This dissertation establishes a vertically related model to discuss when an upstream firm in the third country provides intermediate goods to two differentcountries’s downstream firms, the impact of trade liberalization on upstream firm’spricing and the two countries’ social welfare, If one downstream firm want to sell it’s product in the other country. Our findings are as follows. When the upstream firm adopts uniform pricing and the tariff is not very high,the upstream firm will reduce the price of intermediate goods to induce firm Aexport it’s product to country B. When a lower tariff makes firm A sell it’s productin country A and B, the consumer surplus of the exporting country and the firm’sprofit of the importing country may increase. In addition, the social welfare of importing country must be higher than the case of autarky. When the upstream firm adopts discriminatory pricing, and the market size oftwo countries are similar, the consumer surplus of the importing country may inversely decrease, if trade liberalization makes firm A sell in the other country, but the firm’s profit of the importing country will certainly decrease. By the reason, the social welfare of the importing country in the case of autarky may be higher than the case that firm A sell it’s product in the country B.