As main frameworks for marine environmental cooperation in the East Asia, two programmes can be identified-NOWPAP (North-west Pacific Action Plan) and PEMSEA (Partnerships in Environmental Management for the Seas of East Asia). While NOWPAP, one of UNEP's Regional Seas Programmes, emphasizes reform of environmental legislation and information database, PEMSEA, with aim to achievement of sustainable development of the Seas of East Asia, has implemented various projects to incorporate environmental objectives in all stage of economic development. PEMSEA has projects to promote Integrated Coastal Management (ICM), environmental risk assessment and management, capacity building, partnerships among various stakeholders, etc. In this study, following the description on the implementation status of activities under PEMSEA, the needs for a comprehensive assessment framework for the programme are identified. Then, this article presents a method to evaluate effects of projects implemented under PEMSEA in quantitative manner, because the existing evaluations of PEMSEA activities have focused on qualitative aspects. The presented system model is formulated to include not only environmental data but also socio-economic factors such as GRP, industrial production, activities of marine industry, change of population, etc. Such quantitative evaluation method is useful when each stakeholder examines real benefit of regional marine environmental cooperation and decides on how to commit such activities. Especially, the evaluation model will provide important suggestion toward Japanese environmental diplomacy in East Asia. JEL classification: C61, Q53, Q56