This thesis aims to evaluate the scope of environmental protection in the host states against the states’ obligations to protect and promote foreign investments, and to identify how the existing international investment treaty practice and dispute settlement practice are insufficient in light of considering the environmental interests of the host states in the standards of treatment, including the fair and equitable treatment, national treatment, most-favoured-treatment, and non-expropriation standard. This thesis argues that the existing regime of international investment law does not provide an appropriate framework for the protection of the host states’ environmental interests, especially in the countries with economic and social transition (like China) where the domestic need for environmental protection is emerging and growing significantly. In contributing to the means through which the host states are able to regulate foreign investments without otherwise violating treaty obligation, this research proposes: (1) interpreting investment treaty provisions by introducing more environmental consideration, and (2) rethinking and reshaping the current pro-investor mechanism of international investment law through embracing the provision of broad environmental exception.