DFT-based calculations were employed to investigate mechanisms of 1,3-dipolar cycloadditions between phenylacetylene and an azide (phenylazide or benzylazide) inside carbon nanotubes, whose diameters range from 10 to 14 Å, by obtaining their reaction species (reactant complex, transition state (TS), and product (Pro)). The reactions yield 1,4- and 1,5-triazoles, whose paths are denoted by 1,4- and 1,5-approaches, respectively. We found different geometrical features of reaction species between 1,4- and 1,5-approaches. Reflecting different reaction species, nanotube confinement has the power to enhance kinetically and thermodynamically controlled regioselectivity of 1,3-dipolar cycloadditions to form 1,4-triazoles. In inner 1,4-approaches, the reaction species have planar structures, being small relative to the cavity of tube hosts, and then, their activation energies are slightly lowered relative to those without tube surroundings, independent of the tube diameter. In inner 1,5-approaches, reaction species have phenyl groups overlapping each other, depending on the tube diameter: L-shaped and stacking fashions are found in thick and thin tubes, respectively. Particularly, the stacking fashion in thin tubes results in repulsive orbital interactions between two phenyl rings, destabilizing their TS and Pro. The presence of overlapping phenyl groups increases the activation energies in the 1,5-approaches with a decrease in the tube diameter, being larger than those without tube surroundings.