NIR-Photoluminescence (NIR-PL) spectroscopy is a powerful analytical technique to determine the electronic and molecular structures of semiconducting single-walled carbon nanotubes (SWCNTs). The origin of the observed PL peak is reliably assigned to SWCNTs with specific chiral indexes (n, m) because the emission and excitation spectra show characteristic peaks depending on the molecular structure of SWCNTs. The detailed composition of the bulk samples can be deduced from the PL intensities divided by the theoretically calculated PL yields. We also report band gap modification of SWCNTs by incorporating fullerene molecules (nanopeapods) based on the PL results. It is found that differences in the optical transition energies between unfilled semiconducting SWCNTs and the corresponding C₆₀ nanopeapods strongly depend on the tube diameter and “2n + m” family types. The change in optical band gap is rationally explained by local strain of SWCNTs upon C₆₀ insertion and hybridization of the electronic states between the encapsulated C₆₀ and the outer SWCNTs.