直覺上，跨國資本的自由流動對投資國與被投資國的經濟均有益處，亦可促使資源在國際間以更有效率的方式分配，增進雙方的福利。然而實際上被投資國卻對外來投資加以限制，並且須透過雙邊協商的方式移除此限制。因此，前述自由投資增進兩國福利的直覺似乎與既存的眾多雙邊投資協定相互矛盾。本文建立一基本的理論模型，考慮直接投資以及兩國的策略性投資政策，藉以說明 Nash Equilibrium 為兩國相互課稅，且無單邊降稅的動機。因此，只有透過雙邊簽署投資協定的方式共同降稅，方能消除課稅所產生的無謂損失。 / Intuitively, the free mobility of transnational capital not only benefits home countries and host countries, but also allocates resources globally in a more efficient way, which makes their welfare increase. However, host countries actually implement many restrictions on cross-border capital and try to remove these through bilateral negotiations. Therefore, the intuition that free investment between two countries will increase their economic welfare seems to be contradictory to many existing bilateral investment treaties (BITs). This article provides a theoretical model with foreign driect investment (FDI) and strategic investment policies, first, as to explain the Nash Equilibrium is that two countries will tax investors' FDI behavior. Second, it explains both countries do not have any motivation to reduce taxes unilaterally. Therefore, only when these two countries decide to remove all restrictions on foreign capital mutually by signing bilateral investment treaties do they eliminate the deadweight loss which restraints bring about.