As environmental concerns have been raised from the different aspects of economic activities, protecting the environment may somehow have some effects on the private parties' interest. In the international investment law field, there is also a potential tension between the foreign investment and environmental protection. On the one hand, international investment agreements provide full and effective protection to foreign investors for promoting investment. On the other hand, the adoption of environmental measures by the state will inevitably interfere with the foreign investment. The thesis starts with assessing the relationship between foreign investment protection and the environment. After having a clear understanding of the relationship between the two competing rights, the next step is to take a balanced approach to assess the conflict between foreign investment protection and environment. As there are plenty of international investment cases related to environmental issues, the thesis then analyzes how the arbitration tribunal assess of those issues. During the contextual assessment, the policy power doctrine and proportionality principle have played an important role. From the procedural approach, the participation of Amicus Curiae is also an effective way to balance the public interest and private rights. At last, from the perspective of the IIAs practices, the thesis assesses and proposes incorporating CSR standards and environmental provisions into the IIAs to balance the foreign investment protection and environmental protection. Keywords: Environmental Protection, International Investment Law, Public Interests, Proportionality Principle, Amicus Curiae, CSR