This study analyzed elementary school students’ understanding of floating and sinking, focusing on the generalization of their understanding to extremely large or small objects and the effect of perceptual information. In terms of consistency in the judgement of floating and sinking based on kinds of materials, the results showed that 1-2 graders were more affected by the existence of perceptual information, less likely to cite density or material traits for their explanation, and less likely to generalize their understanding to extremely large or small objects as compared to 5-6 graders. However, even among 5-6 graders, just a few tended to show consistent judgement on floating and sinking based on density or material kinds across all items, including extremely large or small objects. Beginning from the object with prominent perceptual traits, and followed by familiar objects, they are considered to gradually generalize their understanding of floating and sinking based on material kinds. The results clarify the developmental course suggested in previous research, by directly asking for children’s judgements on density-related phenomena, including extreme cases or cases without certain perceptual information.