This study aims to investigate how primary school students' three-dimensional geometric thinking changes across grades. The survey model was used, and the study group was comprising of 520 primary school students in a large city of Turkey. In the study, the Three-Dimensional Geometric Thinking Test, which is a paper and pencil test, was used to collect data. The scores taken from the test were compared across the grades and the relationships between the components of three-dimensional geometric thinking were examined. The findings showed that as the grades increased, students' scores taken from the Three-Dimensional Thinking Test also increased significantly. Moreover, a medium and positive correlation was found between the components of three-dimensional geometric thinking. The results of the study revealed that grade level is a significant variable on three-dimensional geometric thinking, yet some important three-dimensional geometric thinking skills can be developed independent from the grade level. The current study intends to shed light on the development of three-dimensional geometric thinking starting from early grades, and to provide important information for organizing the three-dimensional geometric content in the curriculum and its implementation.