The rotational barrier and conformer energies of butane are well-known, but the contributing effects to its conformational isomerism are still unclear. Calculated potential energy surfaces for the relaxed and vertical (bond distances and angles frozen) structures, together with NBO analysis, suggest that approaching or distancing methyl groups involve substantial energy costs between the gauche and anti isomers, while hyperconjugative interactions play an important, but not prevalent, role for the conformational isomerism of butane.