Abstract. The aim of this study was to understand preservice teachers' changes in their perceived teaching efficacy and theories of mathematics teaching by repeated field-teaching. A mathematics (teaching) methods course was designed to include repeated field-teaching with multiple supports, including course instructors, school teachers, peers, and a course website. The course website was used to organize all course reading materials, discussion, work submission, and research data collection. Compared with that after first filed-teaching, the preservice teachers' teaching efficacy increased after the second field-teaching in pedagogy-content coordination, fluent teaching process, effective solution to student difficulties, and problem posing. In addition, their theories of effective mathematics teaching changed from teacher-centered, theoretical, and procedural approaches to student-centered, practical, and thematic ones.