This study investigated the effect of a paper folding activity prepared to develop the sixth-grade students' concept definitions and images of parallelism and perpendicularity concepts. The study also examined how the concept definition and images changed after the paper folding activity. A combination of quantitative and qualitative methods was used. A one-group pre-/posttest design revealed that the paper folding activity had a significant positive effect on students' concept definitions and images. In addition, the interviews after pre- and post-tests indicated that the students' personal concept definitions of parallelism and perpendicularity of two lines/line segments began to match the formal concept definitions of these concepts after the paper folding activity. Lastly, missing and mis-in concept image situations, encountered generally in the pre-test, were observed less after the paper folding activity.