Radial-breathing mode (RBM) frequencies of armchair nanotubes encapsulating C-60 molecules (peapods) are calculated on the basis of the local density approximation in the density functional theory. The RBM frequencies of peapods exhibit unusual shifts from those of corresponding nanotubes depending on the space between the walls of nanotubes and C-60. Encapsulation of C-60 molecules is found to cause a higher frequency shift for (10, 10) nanotube but a lower shift for (11, 11) and (12, 12) nanotubes. This diameter dependence of the frequency shift is ascribed to the charge redistribution caused by weak hybridization between pi-electron states and nearly free electron states. (c) 2007 Elsevier B.V. All rights reserved.