The influence of the substitution positions of donors and acceptors on quadratic hyperpolarizabilities (&bgr;) was simulated by the AM1 method using four types of conjugated long‐chain molecules having polydiacetylene structures. The chain‐length dependences of &bgr; are greatly affected by the molecular type. The molecule, in which donor‐donor pairs and acceptor‐acceptor pairs alternately appear in the chain, has a narrow energy gap of about 1.6 eV and an exceptionally large &bgr; per 10 A˚ chain length, about 110 times larger than &bgr; of 2‐methyl‐4‐nitroaniline.