This study examines the collaborative professional development (CPD) of three pairs of mentor teachers and pre-service teachers in a junior high school. Of particular focus is the integration of technology into instruction, by using technological pedagogical and content knowledge (TPACK) to evaluate professional development. A qualitative research method based on classroom observations and focus group interviews (FGIs) is adopted. Additionally, data obtained from instructional plans, video-recorded classroom observations and FGIs are analyzed using a constant-comparison analysis method. Analysis results indicate that mentor teachers adjust their instruction methods when they receive the support of pre-service teachers specifically by moving from presenting technological content knowledge (TCK) to constructing various TCK bases. The pre-service teachers constantly apply TCK and technological pedagogical knowledge (TPK) to develop professionally, especially in TPACK-related technology integration concepts. Notably, the CPD program benefits pre-service teachers more than mentor teachers because the former actively seek more opportunities to apply TPACK than the latter, who simply exchange digital instructional materials.