We have investigated the structural dependence of static second hyperpolarizabilities (γ) of polycyclic diphenalenyl radicals, i.e., IDPL, as-IDPL and TDPL, using the hybrid density functional theory method. It turns out that the diradical character of as-IDPL (0.923) is larger than that of IDPL (0.770) and is close to 1 (pure diradical state), which supports the fact that the γ of as-IDPL (472 × 103 a.u.) is smaller than that of IDPL (2284 × 103 a.u.). The replacement of a central benzene ring (as-IDPL) by a thiophene ring (TDPL) increases the γ (1375 × 103 a.u.). This is predicted to be caused by the reduction of aromaticity of the central ring and by the hypervalency of the sulfur atom of the thiophene.