The aim of this study was to explore the degree to which a professional development (PD) program designed using GeoGebra influences a group of Turkish middle school teachers' beliefs in relation to mathematics and role of GeoGebra in mathematics education. In order to collect the required data, the PD course was established to provide six teachers with an opportunity to obtain practical experiences with and necessary knowledge about the use of GeoGebra in mathematics education. In this context, a multiple case study approach was utilized, in which mathematics teachers were observed and interviewed using a variety of procedures to expose qualitative data about their beliefs and the dynamics of their beliefs changes during the course. The main findings illustrated that teachers' beliefs were transformed to some extent in favour of the use of GeoGebra, as well as using constructivist ideas in their mathematics teaching and learning can be attained through the GeoGebra based PD course. In fact, the relationship amongst stated beliefs and intended teaching practice is complex and the social issues were very influential on teachers' pedagogical decisions. This research also introduces some suggestions for researchers who focus on professional development of mathematics teachers relative to their beliefs.