The purposes of this research were 1) to study the current situation and problems in developing core competencies and functional competencies of teachers in private schools under the Office of Education, Chaiyaphum province and 2) to develop guidelines for developing core competency and functional competencies of teachers in private schools under the Office of Education, Chaiyaphum province. The sample group consisted of teachers in private schools under the Office of Education, Chaiyaphum province, academic year 2022, with the total number of 248 people. The sample size was determined by using Krejcie & Morgan table. The tool used in this research was questionnaire about the current situation and problems and guidelines for developing core competencies and functional competencies of teachers with reliability at 0.90. The statistics used for data analysis comprised percentage, mean and standard deviation. The data were analyzed with a statistical package program, guidelines were analyzed with frequency and descriptive analysis. The research results were found that: 1. The current situation in developing core competencies and functional competencies of teachers in private schools under the Office of Education, Chaiyaphum province, in overall, was rated at a high level of practice. The problems in developing core competencies were rated at a low level and the problems in developing functional competencies of teachers were rated at the lowest level. 2. Guidelines for developing core competencies and functional competencies of teachers in private schools under the Office of Education, Chaiyaphum province were concluded consisting of 49 guidelines, divided into 20 guidelines for core competencies, namely 1) achievement-oriented performance with 3 guidelines, 2) good service with 2 guidelines, 3) self-development with 3 guidelines, 4) team-working with 4 guidelines, and 5) moral and ethics in teaching profession with 8 guidelines, and 29 guidelines for functional competencies, namely 1) curriculum and learning management with 5 guidelines, 2) learner development with 5 guidelines, 3) classroom management with 5 guidelines, 4) analysis, synthesis and research for learner development with 3 guidelines, 5) teachers’ leadership with 7 guidelines and 6) building relationships and cooperation with communities for learning management with 4 guidelines.