AbstractSingle‐wall carbon nanotubes (SWCNTs) that are encapsulating C60 [nanopeapods (NPDs)] can be considered one of the best model systems for investigating host–guest interactions in π‐conjugated molecular systems. We thus have investigated the effects of C60 encapsulation on the radial breathing mode (RBM) frequencies of semiconducting SWCNTs in a wide range of diameters (dt = ∼1.23 − 1.5 nm). The observed frequency shifts show a characteristic behavior depending on the intermolecular spacing between encapsulated C60 and the host SWCNTs. The present findings clearly indicate the van der Waals nature of the SWCNT‐C60 interaction.