The effects of C(60) encapsulation on the radial breathing mode (RBM) frequencies of single-wall carbon nanotubes (SWCNTs) are investigated over a wide range of diameters (d(t) approximately 1.25-1.5 nm). The observed frequency shifts show a characteristic behavior depending on the inter-spacing between C(60) and SWCNTs. The present findings clearly indicate the van der Waals nature of the SWCNT-C(60) interaction and an importance of hybridization between the electronic states of C(60) and SWCNTs.