We report on cathodoluminescence (CL) of monosized and well-aligned 4 Å single-walled carbon nanotubes in a zeolite template (AlPO4 -5 single crystal). The CL exhibits three emission bands centered at 1.87, 2.22, and 2.98 eV, which are assigned to three possible 4 Å tube structures (4,2), (5,0), and (3,3), respectively. The assignation is based on first-principles calculations, Raman scattering measurement, and CL behavior of various samples. The emission peak shift of the (5, 0) tube is explained by the transition mechanism being different from photoluminescence due to the excitation of electrons with high energy. © 2005 American Institute of Physics.