This study aimed to investigate secondary mathematics teachers' noticing of students' mathematical thinking in the context of a professional development (PD) program utilizing modeling-based teacher investigations. The PD program was conducted with two groups of mathematics teachers from two public high schools. It included seven 1-month-long cycles, each consisting of an introductory meeting, the teachers' classroom implementations of a model-eliciting activity (MEA), and a follow-up meeting focusing on the teachers' collaborative investigation and discussion of students' written work, including their strategies to solve the MEA. Findings suggested that participating in the PD program promoted the teachers' noticing of students' mathematical thinking, albeit differently, over time. We elaborated on the components of the PD program that might have influenced the teachers' noticing of students' mathematical thinking. Implications of the findings on teachers' noticing for researchers, PD leaders, and teacher educators are discussed in terms of potential directions for future research and practice.