Nonlinear optical susceptibilities of a new SHG (second harmonic generation) material, 3-aminoxanthone, were investigated at both molecular and crystalline levels, by using the ab initio CPHF (coupled perturbed Hartree-Fock) method and the oriented-gas approximation. Molecular first hyperpolarizabilities beta of 3-aminoxanthone were analyzed in order to investigate the source of beta, by using an intramolecular oriented-gas approach which assumes the molecular beta can be obtained by making a tensor sum of the beta of molecular fragments. The analysis revealed the source of beta is localized on one aromatic ring, and effective pi-conjugation for beta is almost completely disconnected between the aromatic rings. The results of the oriented-gas calculations showed the crystal has a phase matching condition of SHG, and its chi(16)((2)) is 10.6 times that of the chi(14)((2)) of urea crystal; these results are in agreement with the results of the SHG powder efficiency measurements. Experimental refractive indices were needed for the local field correction of the oriented-gas approximation in order to estimate the experimental order of magnitude of chi((2)). The localized source of beta may provide a new way for molecular design of organic nonlinear optical materials.