Geometrical and electronic features of the armchair C40+20n and zigzag C42+18n nanotubes capped with fullerene hemispheres, in which n is the number of cyclic cis- and trans-polyene chains involved in the nanotubes, are analyzed from density functional theory (DFF) calculations. To illuminate the role of the fullerene hemispheres in their structures, the corresponding nanombes terminated by H atoms, C20nH20, and C18nH18, are also studied in terms of orbital interactions. Although there are various bond-deformation patterns within the cis-polyene chains in the C20nH20 series, the C40+20n series always have similar bond-length alternate patterns, in which the C-C bonds along the tube axis are shortened, whereas the C-C bonds around the belts are lengthened. The bond- deformation patterns in the C42+18n series are also different from those in the C18nH18 series; the structures of the capped and uncapped zigzag nanotubes exhibit Kekule-and Clar-networks, respectively. The geometrical change in the capped nanotubes is a consequence of orbital interactions in the cylindrical segments with the fullerene hemispheres. Accordingly, the fullerene hemispheres play a dominant role in determining the geometrical and electronic properties of the capped nanotubes. DFT calculations also demonstrate that the geometries of the armchair series exhibit an oscillatory behavior as the chain width with a periodicity of 3. The interesting nanometer size effect is reasonably understood from significant interchain interactions between the cyclic cis-polyene chains involved. In contrast, the geometrical features in the zigzag series are irrespective of the chain width, due to nodal properties of the single trans-polyene chains.