Static second hyperpolarizabilities (gamma’s) for para-nitrophenyl nitronyl nitroxide (p-NPNN), which is one of thermally stable organic radical systems, are calculated by the finite-field (FF) method in the INDO coupled unrestricted Hartree-Fock (CUHF) approximation. The hyperpolarizability density analysis is performed to elucidate the characteristics of spatial contributions of electrons to the gamma’s. It is found from these results that the largest component (gamma(xxxx)) of this system is negative and is dominantly determined by the contribution from the gamma(xxxx) densities of nitronyl nitroxide radical group.