The geometry textbooks of the new system of secondary education in Iran differed dramatically from the old ones considering the aims, the visions, the content, the approach, and the educational purposes. Four hundred eighty mathematics teachers participated in a nationwide professional development program conducted by the author to facilitate the implementation of the new changes. Based on this program, a research study was designed to investigate the teachers' beliefs about a new reform in high school geometry as a result of the professional development program. The data of the study were collected through teachers' reflective writings, project works, group discussions, reflective questionnaires, and oral communications. The purpose of this paper is to draw upon this study and to document mathematics teachers' beliefs about new changes.