Illegal wildlife trade threatens Southeast Asia’s remaining biodiversity and natural ecosystems. Drastic decline in populations of high commercial value species such as tigers is inevitable. Tigers are an umbrella species. Saving tigers means saving everything which connect to them, including our healthy ecosystem. Tigers are mostly used for ingredients in medicines and food. The increasing demand of tigers, tiger parts and its derivatives has triggered the illegal trade of tiger. The illegal trade of tiger involves a complex network from source, transshipment, market and to the final consumer. It is able to move across the borders quickly and involves more than one state. This undergraduate thesis aims to explain why cooperation in ASEAN WEN is needed to combat the illegal trade of tiger in Southeast Asia. International environmental regimes and multilateral cooperation are the theory used to explain those issues.