We investigate the intermolecular-interaction effects on the longitudinal second hyperpolarizabilities (gamma) for pi-pi stacking dimers of pi-conjugated cation monomers, C5H7+. Three types of geometrical configurations are examined to elucidate the relation between the longitudinal gamma and the pi-pi orbital interaction in the perpendicular direction to the monomer plane. The gamma values are calculated by the finite-field (FF) approach using several ab initio molecular orbital (MO) and density functional (DF) methods. The spatial contributions of electrons to gamma are analyzed using the plots of the second hyperpolarizability (gamma) densities, which are described by the third-order derivatives of the charge densities with respect to the applied electric fields. It is found that the relative phase relation between interacting upper and lower pi-orbitals is closely related to the intermolecular-interaction effects on gamma of a dimer. Namely, in-phase (bonding-like) pi-pi orbital interactions in the HOMO and HOMO-1 levels of a dimer tend to decrease the magnitude of gamma density and then reduce the contribution to gamma at the interacting sites. On the basis of this result, we discuss a novel guideline of controlling third-order nonlinear optical properties.