This article reports on reforms of the science education program for prospective physical science teachers in Taiwan and examines their impact on these teachers. The reforms include restructuring the courses in physical science methods, and requiring all prospective teachers to do action research during their one-year full-time teaching practice. The former aims to instil knowledge of constructivism and enable them to teach physical science by using constructivist approaches, and the latter provides them with an opportunity to apply what they have acquired to actual teaching. This study shows that exposure to constructivism affected the teachers' beliefs and initial choice of science teaching approach. Evidence further demonstrates that successful experience with action research reinforced their beliefs, and made them more willing to employ constructivist science teaching approaches in their teaching practice. Introducing these reforms in Taiwan has the potential of enhancing the competency of prospective science teachers. (Contains 4 tables.)