This study is based on the perspective that participations and confrontations of NGOs to the global issues are meaningful since 1990s. Focusing on transboundary environmental issues in Mekong region, it looks into the traits, type, and strategies of transnational activities of local NGOs working in Mekong region. First of all, their transnational activities are approaching from not only regional but also socio-cultural perspective beyond nation-centered and economic perspective. And they are getting the initiatives of their transnational activities instead of depending on high-reputed international NGOs. For doing it, as a main strategy, they are establishing networks for close cooperation with local NGOs based on Mekong region whenever they need that. In this study, the case of Thailand's NGOs in environmental sector is explained to show their traits, type, and strategies of transnational activities. However, despite their visibility in Mekong region, there are limits to influence on decision making process of environmental governance by nation and economic centered perspective. To lessen these limits, enlargement of political freedom and the political space of civil society and public direct participation to the political process are necessary in the individual states in Mekong region.