This study emerged from the concerns experienced by the last-year English language trainee teachers during their school practicum. An increasing number of trainees complained that their existing beliefs conflicted, in many ways, with the school-based mentor's teaching practice. A collaborative action research (CAR) professional development programme (PDP) was established to help prospective teachers resolve many of the dilemmas and improve their classroom practices in a 10-week practicum course. It was found that CAR has a powerful impact upon teacher candidates as it solves many of the dilemmas and concerns. Belief changes of one trainee teacher are presented as an exemplary case. While such findings can improve our understandings of pre-service teachers' cognitive learning and problem solving skills at the practicum site, they also generate useful insights into designing a PDP to promote trainee teachers' school-based professional development in STEM (science, technology, engineering, and mathematic) education.