Geometry is the study of shape and space. Without spatial ability, students cannot fully appreciate the natural world. Spatial ability is also very important for work in various fields such as computer graphics, engineering, architecture, and cartography. A number of studies have demonstrated that technology has an important potential to develop spatial skills. In the present study, the effect of dynamic geometry software (DGS) Cabri 3D on student mathematics teachers' spatial skills was examined. The study used a one-group pretest-intervention-posttest experimental design. In this context, 40 student mathematics teachers took "Purdue Spatial Visualization" (PSV) test as pretest at the beginning of the study, and then some implementations were carried out with these students by using Cabri 3D software for 8 weeks. The PSV test was retaken by students at the end of the study. That if there was a meaningful difference between the pretest and posttest results was examined by using paired sample t test. The findings demonstrated that these computer supported activities contributed to development student mathematics teachers' spatial skills. (Contains 3 table and 8 figures.)