在同時有區域性污染及跨界污染的情況下，本文利用Brander and Spencer（1985）的三國兩廠商出口模型來分析開放經濟體系下出口國之最適環境政策與進口國的最適關稅政策。本文發現：一、若本國遭受外國排放的跨界污染，此污染造成本國環境的邊際損害相對小（大），則本國應對本國出口商課環境稅（補貼）；二、若本國對外國排放的跨界污染，其邊際損害相對大（小）於外國對本國排放的跨界污染的邊際損害時，則本國應對本國廠商課環境稅（補貼），而外國應對其廠商補貼（課稅）；三、進口國課徵之關稅大小不但取決於出口廠商的生產成本，也取決兩出口廠商排放區域污染的邊際損害、以及跨界污染的邊際損害。

The paper employs a duopolistic model to analyze the optimal environmental policy for an exporting country given that the exporting firms may emit local pollution and transboundary pollution to the neighboring countries. We find that the optimal environmental policy for the exporting country is to levy tax on firms that only emit local pollution; whereas it is optimal to subsidize (tax) the exporting firms that may cause local and transboundary pollution in the circumstance that the marginal damage of transboundary pollution is relatively large (small). Furthermore, the importing country should impose a higher tax on the higher-cost exporting firm if the higher-cost exporting firm emits pollution that causes relatively smaller marginal damage to the importing country.