The third-order optical non-linearity was measured in nanopolyacetylene using ferntosecond optical Kerr effect technique at wavelength from 790 to 860 nm. The ultrafast non-resonant effective second-order hyperpolarizibility for single polyacetylene nanoparticle was estimated to be as large as 1.0 x 10(-27) esu, which corresponding to the y(c) of single carbon atom contribution equals to 5.0 x 10(-33) esu. We ascribed the observations to the highly ordered structure of nanopolyacetylene and delocalization of the pi electrons along the conjugated chains was supposed to be responsible for such properties. (C) 2002 Elsevier Science B.V. All rights reserved.