It is recognized that strategic environmental assessment enables public policy makers and other stakeholders to take environmental impact into consideration early in policies, plans, and programmes deliberation and formulation processes. Meanwhile, public participation is the cornerstone to enhance sustainability of policies, plans and programmesformulation, and strategic environmental assessment. However, a question has often been raised: 'Why is public participation so important but often ignored in strategic environmental assessment?' This research, on the basis of legal reasoning method, conducts an exploration of theory and practiceregarding public participation in strategic environmental assessment. On one hand, it discusses the theoretical basis of public participation and the importance of carrying out public participation in strategic environmental assessment. On the other hand, it reviews thirteen strategic environmental assessmentprojects in Taiwan, and identifies shortcomings and challenges to conduct public participation in strategic environmental assessment. Finally, this research proposes how regulation can serve as a lever-like tool to enhance public participation in strategic environmental assessment and takes Taiwan as an example.