Identifying the key reaction sites in supported-metal nanoparticles (NPs) is critical in designing high-performance catalysts for industrial chemical reactions, such as catalytic conversion of methane (CH4). However, the high-temperatures (> 400oC) required for CH4 conversion cause the metal NPs to aggregate into larger crystallites, and thus make the direct relationship between structure and function in these catalysts difficult to identify. Here we overcome this issue by designing a post-encapsulated composite structure in which individual Pt NPs are surrounded by gas-permeable and catalytically active CeO2 shell. In particular, we analyze the catalytic activities for CH4 combustion of the encapsulated catalysts by varying the size of the Pt cores precisely and thus changing the metallic surface and metal-oxide interface site densities. The specific surface area of the Pt exposed to gas in each particle is obtained through chemisorption analysis, and the Pt/ceria interface density is reliably deduced using the CO oxidation as a reference reaction. Based on these observations, we succeed in unambiguously identifying the CH4 combustion occurs actively at the Pt/ceria interface. Our results suggest a reliable approach to explore the active sites for various high-temperature catalysis.