The ab initio elongation finite-field method is applied to four ladder-structure polydiacetylenes derivatives to investigate the effects of arrangements of donors and acceptors on their nonlinear optical properties. All four systems have large (hyper) poarlizabilities. It is found that the same-direction electron transfer path perpendicular to the molecular axis of ladder-type polydiacetylenes can increase their first hyperpolarizabilities, while the electron transfer path parallel to the molecular axis can effectively increase their second hyperpolarizabilities. It should be mentioned that the random arrangement of donors and acceptors should be avoided in designing the nonlinear optical materials like polydiacetylenes. (C) 2009 Elsevier B. V. All rights reserved.