This research aimed to determine how the beliefs of high school students about mathematics changed in the environment enriched by dynamic mathematics software. The research was designed according to the pretest-posttest single group model which is one of the weak experimental research designs. The beliefs were determined by a belief scale which has three sub-dimensions before and after a 20 weeks elective mathematics course supported by the dynamic mathematics software GeoGebra. After the course, while some items about mathematics learning processes has been positively changed significantly some of the items showed a positive change even if it is not significant. A significant change, about nature of mathematics and usage of mathematics, could not be determined. According to these findings, it is thought that mathematics courses conducted by dynamic mathematics software have the potential of affecting the mathematical beliefs positively.