This dissertation is an analysis of the legality of such unilateral measures under international law. One specific incident will serve as a detailed case study: the US's import ban against wildlife products of Taiwan authorised by the Pelly Amendment in 1994. This action constituted an unprecedented imposition of unilateral trade sanctions to pursue an environmental object, the preservation of endangered species. There will first be a discussion of the facts and background relevant to each case at the preliminary stage. The research will then assess the legality of this measure from several standpoints: general rules of international law, third-State countermeasures, international environmental law, and international economic law, and international economic law. This study will proceed to explore the legal implications of individual applicable law over the environmental trade sanctions. Finally, in each of these areas, the lawfulness of the Pelly sanctions will be assessed by applying the pertinent rules to the facts. The assessment of the Pelly sanctions leads to the following conclusions: (1) no definite rules have yet emerged to proscribe the use of economic sanctions in the context of general international law, even though it is believed that certain trade embargoes are likely to be in breach of non-intervention doctrine; (2) the Pelly sanctions were not legal countermeasures, mainly because Taiwan violated none of international rules regarding species protection; (3) given the insufficient legal endorsement of the US's sanctions by any international environmental regime, the Pelly measures were incompatible with the consensus process required by the Rio Declaration of 1992; (4) although the Pelly sanctions had not violated the US's obligations under the GATT/ WTO regime because of the non-member status of Taiwan, a hypothetical study shows that the sanctions would have been inconsistent with GATT mandate if Taiwan had been a member, (5) In terms of the FCN treaty between the two countries, US could meet substantial difficulty in justifying the trade measure.