We investigated the effect of density fluctuation of supercritical carbon dioxide (scCO(2)) on anomalous swelling of multilayer polymer thin films On the ridge in the pressure temperature phase diagram of CO2. In order to measure the swelling ratio along the film depth, we alternatively deposited hydrogenated poly(methyl Methacrylate) (PMMA) and deuterated polystyrene (dPS) thin films and performed the neutron reflectivity measurements as a function of CO2 pressure at 36 degrees C. The results showed that, in contrast to previous studies, CO2 was to penetrate deeply:throughout the, multilayer thin film where the magnitude of swelling along the density fluctuation ridge of CO2 was independent of film thickness. Block copolymer thin films of dPS-b-PMMA with a parallel lamellar orientation also showed similar swelling behavior in scCO(2). However, it is well-known that single-layer polymer thin films exhibit anomalous swelling behavior only the film surface. This difference is probably due to the fact that the multilayer thin films have the CO2-philic PMMA layer sandwiched between dPS layers, which can function, as a CO2 reservoir, thereby transferring the CO2 molecules from the PMMA layers to the adjacent dPS layers. Furthermore, we found that the interaction between polymers and substrates was not significant in scCO(2) from diffusion dynamics results using neutron reflectivity, thereby facilitating anomalous dilation of polymers near the substrates without a pinning effect.