Similarity is a crucial concept introduced in the 8th grade, and it is further developed and reinforced through associations during high school. This research aims to investigate how 8th grade students construct the concept of similarity using concrete manipulative support within the framework of APOS theory. The research was conducted as a qualitative case study and the learning environment was supported by a concrete manipulative (pantograph), A readiness test was administered to 15 students from a central public school in the Black Sea region to assess their prior knowledge of topics related to similarity. Based on the results, four participants with varying levels of understanding were selected. Data collected from the test, observations, and individual interviews were analyzed through content analysis. Findings indicate that the participants could construct the concept of similarity in a concrete manipulative-supported environment (pantograph) efficiently and meaningfully. The results show that all participants internalized the process by constructing similar polygons with the pantograph and understood that the corresponding side lengths of polygons with equal angles have the same ratio. However, while most participants reached the object stage, the participant with the lower intermediated level struggled to coordinate concepts like ratio, angle, and measurement, making it difficult for him to encapsulate the concept and reach the object stage. [This paper was published in: "EJER Congress 2024 International Eurasian Educational Research Congress Conference Proceedings," edited by Senel Poyrazli, Ani Publishing, 2024, pp. 157-165.]