This paper presents the process and the results of the development of a mentoring model based on pedagogical content knowledge (PCK) and lesson study, called the ADD-CoRE model, which is valid, practical, and effective. This was Research and Development research. This model was developed through the collaboration between the lecturers at Mathematics Education Study Program and junior high school mathematics teachers in Palopo City. The specific goal of this study was the availability of the ADD-CoRE model syntax, supplemented by supporting tools, namely the model book and the textbook. The ADD-CoRE model development process consisted of four phases, namely the initial assessment, the design, the realization, and the testing, evaluation, and revision phase. The components included in the model were: syntax, social system, the principle of reaction, support system, instructional impact and accompaniment. Field trials were carried out three times at three junior high schools in Palopo City. The results of the development of the teacher mentoring model based on PCK and lesson study (ADD-CoRE models) met the validity criteria, as indicated by the validator assessment results. It also fulfilled the practicality criteria based on the teachers' responses, the model implementation, and the PCK teacher observation results. Besides, it satisfied the effectiveness criteria based on students' responses, students' activities in learning, and students' learning outcomes for the topic of polyhedra. The ADD-CoRE model syntax consisted of six phases, namely: (a) Analyzation, (b) Direction, (c) Demonstration, (d) Collaboration, (e) Rotation, and (f) Evaluation. This research is expected to be a mentoring model for teachers and prospective teachers to improve the quality of learning.