The molecular orientation effects on the spectra of gamma(-omega; omega, omega, -omega) (degenerate four-wave mixing (DFWM)) related to two-photon absorption (TPA) are investigated using the J- and H-type dimer models composed of three-state monomers by the numerical Liouville approach. The features of TPA peaks of imaginary gamma turn out to be strongly correlated to the relative monomer orientations. The significant enhancement of TPA peaks of dimers is predicted to be realized due to the double resonance effects regarding one- and two-photon absorption peaks by only adjusting the relative monomer orientations. (c) 2007 Elsevier B.V. All rights reserved.