We discuss total energy electronic structure calculations of the energetics of encapsulation of C-60 in the small diameter carbon nanotubes (15, 0) and (15, 0) -( 24, 0) with diameters 11.8 and 18.7 angstrom, respectively. We find that DWNTs allow for a less endothermic encapsulation by as much as 1.0 eV thus, indicating that encapsulation of fullerenes is facilitated in DWNTs. This is ascribed to the role of the interlayer state which acts as a repository of electronic charge to minimize the electrostatic repulsion. Thus, together with diameter of tubes and sizes of fullerenes, the number of walls of carbon nanotubes is an important parameter to assess stability of carbon nanopeapods. (C) 2008 Elsevier B.V. All rights reserved.