One of the goals of environmental education is to develop environmentally literate citizens who are equipped with awareness, knowledge, attitudes and skills to make informed and responsible decisions and participate in resolving environmental challenges. Pre-service teacher education is one avenue to promote environmental literacy, hence, there is a need to adequately train pre-service teachers so they can achieve the professional competencies of an environmentally educated person and educator. This study aimed to assess the pre-service students’ level of awareness and understanding and their level of competence on environmental education concepts and principles after mainstreaming environmental education in the teacher education curriculum of the 3NS institutions in the Philippines. A developed curriculum guide for Physical Science and lesson plans were implemented over a 9-week period and an assessment tool was used to determine the students’ level of understanding and competence on environmental education concepts and principles. Results of the study showed that the 3NS pre-service students’ know and understand very well the environmental education concepts and principles and obtained a satisfactory level of competence of environmental education concepts and principles. Students also gained positive attitudes towards the environment and developed better awareness and understanding of the environment. However, due to the satisfactory level of competence obtained by the students, there is still a need to further enhance the mainstreaming of environmental education in the teacher education curriculum in order to get more positive results.