The curriculum for Turkish primary mathematics education was revised in 2005. The idea behind these reforms was to move from a subject-centred model to a student-centred model. The aim of this study was to investigate changes in teachers' mathematical beliefs following a professional development (PD) course based on the use of Geogebra. This course was designed to provide teachers with a better theoretical and practical understanding of mathematics teaching and learning through interacting with computer-based mathematical activities. In order to encourage mathematical thinking beyond the routine, participants worked on investigational mathematical tasks through small group exploration, discussion and negotiation, by using Geogebra. Six mathematics teachers, all teaching pupils aged 12-14, participated in seven workshops. The content of mathematical activities was based on areas of the Turkish mathematics curriculum and the Geogebra website. Results of the study showed that participants were inclined to maintain or strengthen their beliefs throughout the course in favour of the child-centred view.