This qualitative study aimed at exploring whether students' successful use of analogy in learning curriculum complex science concepts was related: (a) to the level of their understanding of a specific analogy and (b) to their metacognitive awareness of how the analogy was to be used and of the changes produced in their own conceptual structures. In implementing a biological curriculum unit, students' prior knowledge has been taken into account in order to examine its conceptual growth and change via a not completely introduced analogy to 15 fifth graders as they were engaged in understanding the ways in which the new concepts (on photosynthesis) were similar to a familiar source (making a cake). Qualitative data present the children's mapping processes in elaborating the analogy and their metacognitive awareness of the meaning and purpose of the analogy itself, and their personal use of the analogy in changing initial conceptions. As hypothesised, the results showed a high positive correlation among the level of conceptual understanding of the new science topic, the level of understanding of the analogy, and the level of effective use of the analogy in integrating the new information into the pre-existing conceptual structures. Key implications on the use of analogy for conceptual change in the classroom are outlined.