The cyanine dye (Cy) was embedded in (or absorbed by) a monolayer on air-water interface and its multilayer samples deposited on glass slides were obtained. The studies of their absorption spectra showed that Cy in LB films formed H-aggregates. Multiple phase conjugate waves induced by third- and higher-order nonlinearity of the LB film were observed. High to third-order diffractions with two-beam-interaction and self-phase modulation phenomena with single-beam-interaction in the LB film were also observed. The exceptionally large third-order susceptibility was measured to 5*10-8 esu for the LB film, which is larger than that deduced from measured third-order byperpolarizability of Cy monomers in bulk solution. The origin of this large third-order susceptibility for the LB film, due to the near-resonant and delocalized exiton enhancement in the aggregates, is duscussed as well.