We report optical band gap modifications of single-walled carbon nanotubes upon C-60 insertions by using photoluminescence and the corresponding excitation spectroscopy. The shifts in optical transition energies strongly depend on the tube diameter (d(t)) and the "2n + m" family type, which can be explained by the local strain and the hybridization between the nanotube states and the C-60 molecular orbitals. The present results provide possible design rules for nanotube-based heterostructures having a specific type of electronic functionality.