The study based on the inspiration from Vygotsky’s suggestion utilizing the develop- mental method to investigate qualitative changes of behaviors and analyze the devel- opmental course of young children’s science-related concepts will be presented in this paper. The inquiry-based science project presented is “floating and sinking” con- ducted by a group of 5-year-old children. Science activities from each group were audio-and video-taped for a period of one or two months. Semi-structured inter- viewing of children was conducted to verify current scientific thinking in relation to the science activities children experienced. According to the results, along with the progress of the project, qualitative changes of children’s science-related inquiry behavior as well as conceptual development revealed. The results showed that dis- cussion with the teacher and peers, as well as plenty of time and materials to investi- gate the phenomenon of “floating and sinking”, were beneficial for young children’s conceptual development.