The geometric and electronic structures of porphyrin and a series of carbaporphyrins have been theoretically studied using the time-dependent density functional theory (TDDFT). The two-photon absorption cross sections (TPACS) of these molecules are computed using the ZINDO-SOS formula. The calculated results indicate that when the N atom is substituted by the C atom, the molecular center is enlarged and the absorptions are red-shifted and that porphyrin, carbaporphyrin, opp-dicarbaporphyrin and adj-dicarbaporphyrin show TPACS in small absorption area, while tetra-carbaporphyrin has fairly large two-photon absorptions in comparatively larger area, which may lead it to many practical applications. (C) 2003 Elsevier Science B.V. All rights reserved.