The purpose of the study was to investigate the effects of mathematics instruction supported by dynamic geometry activities on seventh grade students' achievements in the area of quadrilaterals, based on their van Hiele geometric thinking levels. The study followed a nonrandomized control group pretest and posttest research design. Participants of the study were 76 seventh grade students. Students in the experimental group worked in a learning environment supported by dynamic geometry software while other students worked in their conventional settings. The results of the study indicated that there is a significant interaction between the effects of method of teaching and van Hiele geometric thinking levels on students' achievement levels in area of quadrilaterals. In addition, mathematics instruction supported by dynamic geometry activities has significant effects on seventh grade students' achievement in the area of quadrilaterals. Moreover, the results revealed that students in the dynamic geometry supported instruction group received significantly higher scores in the area of quadrilaterals than students in the traditional instruction group when students are at second level of van Hiele geometric thinking.