To achieve the goal of cultivating students' scientific modeling competence, it is crucial to improve teachers' epistemic knowledge and skills with respect to modeling-based teaching. Taking into consideration existing studies on enhancing teaching competence and teachers' professional development on modeling, and the importance of classroom teaching quality for effective and meaningful teaching and learning, we proposed a framework for the assessment of teachers' modeling-based teaching performance (MBTP) in science classrooms. Based on the framework, we developed hypotheses on 18 MBTP indicators and five levels of MBTP, and created a modeling-based teaching observation protocol (MBTOP) to test these hypotheses, to see how well they represent teachers' levels of MBTP, and how well they differentiate teachers with different MBTPs. We found that most of the lessons in our research sample were at a low level, and MBTP indicators proposed in this study could distinguish individual lessons in terms of their levels of MBTP. The validated performance indicators and levels can provide insights for the assessment and development of teachers' modeling-based teaching competencies in science teaching.