This study investigated primary school 5th and 8th graders' understanding and mental models related to the shape of the world and gravity, and how these models reflected the fact and what kind of a change there is from 5th to 8th graders. This research is based on a cross-sectional design. The study was conducted in a low socioeconomic level school and 105 students -52 girls and 53 boys-participated in the study. The data were collected through interviews, one of the qualitative research methods. As a data collection tool, a semi-structured Conceptual Models interview form about the Shape of the World and Gravity (SWGCIF) was used. The results of this study showed that 5th and 8th graders have seven different understandings and mental models about the shape of the world and gravity, one of which was scientific. The scientific approach and understanding were low in both 5th and 8th graders but a slight increase was observed towards 8th graders. In order for a meaningful learning through conceptual change, it can be suggested that these abstract concepts should be concretized by means of visual and audio materials and models. Also, studies with extensive samples should be conducted in order to provide a more comprehensive understanding of the situation.