On the basis of the all-atom model, cohesive energies per carbon atom between two single-walled carbon nanotubes (SWNTs) of uniform and different diameters were calculated, and structural stability of SWNT bundles with differing diameters was analyzed. It is found that the cohesive energy per carbon atom exhibits a minimum when the diameter of one tube is a half of that of the other. So we predict that the SWNT bundle formed by two species of tubes with the diameters of D and D/2 is a novel stable structure. In particular, an energy of 17.44 meV per carbon atom is needed for the separation of a single tube of 2.713 nm from the stable arrangement, while only 15.88 meV per carbon atom for the separation of a single tube of 2.713 nm from the hexagonal array of SWNTs of uniform size, which suggests that the special arrangement structure is more stable than the hexagonal array. It is expected that the calculated cohesive energies per carbon atom may provide valuable information for separation and physical properties of the SWNT bundles with differing diameters. (C) 2007 Elsevier Ltd. All rights reserved.