Thailand formulated a National School Health Policy (NSHP) in 1998, and it has been widely implemented but has not been evaluated. This case study aimed to identify factors that have influenced the implementation of NSHP in Thailand. For this purpose, we conducted a document review and key informant interviews. We selected key interviewees, from NSHP implementers at national, provincial and school levels in four geographical areas. We adopted a content analysis method, using a framework of 12 influential components of successful policy implementation and triangular policy framework. This study showed that NSHP was well-disseminated and implemented at whole country. We identified seven positive factors influencing NSHP implementation, namely matching with ongoing educational strategy, competition and encouragement by an awarding system, sustainable human capacity building at school level, participation of multiple stakeholders, sufficient understanding and acceptance of school health concepts, sharing information and collaboration among schools in the same clusters and functional fund raising activities. In addition, we identified three negative factors, namely lack of institutional sustainability, vague role of provincial officers and diverse health problems among Thai children. The government should clarify the role of provincial level and set up institutionalized capacity-building system as measures to strengthen monitoring and evaluation activities.