The research into the environmental education competency of secondary school teachers was conducted using mixed research methods. The data were collected using the questionnaire and the structured interview. The teachers of eco-schools completed the questionnaire and teacher advisors as well as mentor teachers of the eco-school project participated in the structured interview. The research findings revealed that most respondents had a moderate level of environmental education competency that involved six aspects: knowledge of the environment, basic understanding of the environment, responsibility for the environmental education of professional teachers, planning and practice with regard to the environmental education, promoting learning about the environmental education and evaluation of the environmental education. The approaches to enhance the environmental education competency of secondary school teachers should involve multidisciplinary instruction learning, community-based learning, project-based learning, happy teaching and learning, and holistic learning management.