The third-order optical nonlinearity of the J-aggregates of nonionic dye in vapor-deposited films has been investigated in terms of electroabsorption measurement. Compared with that of a monomer film of the dye, the nonlinearity was enhanced by a factor of 4000. However, the number of aggregates in the J-aggregate film was estimated to be only a few. The obtained enhancement is mainly caused by the concentration of oscillator strength on the J-band and not by the development of the coherence size of excitons. Therefore, a much larger optical nonlinearity can be expected in highly ordered J-aggregates of the nonionic dye, and it is demonstrated that J-aggregate formation is an effective strategy for nonionic dye to improve its optical nonlinearity. (c) 2005 Elsevier B.V. All rights reserved.