We investigate the third-order nonlinear optical property for 1,6,6a-trithiapentalene (TTP) by using density functional method, A unique spatial contribution of pi -electrons to the second hyperpolarizability (gamma) is elucidated. Based on our classification rule of gamma, the features of gamma for other pi -conjugated systems involving sulfur (S) atoms are also discussed from the viewpoint of the possibility of combining conductivity and third-order nonlinear optics.