The absorption and electroabsorption (EA) spectra of 1-methyl-1′-octadecyl-2,2′-cyanine perchlorate (S120) Langmuir-Blodgett films were measured. From these results, dispersion of the third-order nonlinear optical susceptibility χ(3) (-ω; 0, 0, ω) in the range of 1.8 to 2.7 eV was determined. In addition, the linear-polarizability difference Δα and the permanent dipole-moment difference Δμ between excited and ground states were evaluated. Upon comparison with the results of measurement of the S120 monomer film, it was found that the third-order nonlinear optical susceptibility of the S120 LB film is strongly enhanced by a factor of 1000. It was concluded that this strong enhancement is attributed to the fact that S120 molecules with a short methyne chain that tightly binds two quinoline rings form a rigid J-aggregate with a one-dimensional structure that hardly twists, so as to yield a sharp absorption band with a large transition moment. Finally, we evaluated the average aggregation number of the S120 LB film to be 3-4.