Internal rotation of the methyl group in 1- and 2-methylnaphthalenes has been investigated by the ab initio theory. The rotational barriers in the S-0 and S-1 states calculated by the Hartree-Fock and configuration-interaction with single-excitation operator methods are in reasonable agreement with experimental values. The variations of the rotational barriers by excitation (S-0 --> S-1), ionization (S-0 --> C-0), and electron attachment (S-0 --> A(1)) are shown to be directly connected with the stability of the HOMO and/or LUMO by the first-order treatment. In the HOMO and LUMO, a new type of orbital interaction named pi*-sigma* hyperconjugation appears and determines their stability. The interpretation based on the pi*-sigma* hyperconjugation can consistently and comprehensively explain the barrier variations.