This qualitative study aimed at exploring English teachers' understanding of the Basic Education Core Curriculum in Thailand and to investigate the contributing factors. The selected research with triangulation design was used to analyze the data, which was obtained from 66 English teachers. The drawings and questionnaire data showed that most teachers possessed 'impeding understanding', which may interfere with the development of a localized school curriculum. No clear trend was found in the present study on how teachers' professional experiences, teachers' qualifications, professional development activities, and involvement in curriculum development may contribute to teachers' comprehension, as suggested in previous studies. The findings on teachers' lack of understanding of the Core Curriculum after ten years of its implementation indicate the pressing needs to carefully design the implementation of the new national curriculum the Ministry of Education is developing. Effective communication processes and continual in-service professional development activities are recommended. Further investigation on how to effectively provide in-service professional development activities in Thai schools to promote teachers' understanding of a new educational policy will be needed.