We investigated the aggregation status and formation mechanisms of a hemicyanine derivative(HD)in Langmuir-Blodgett monolayers prepared under different conditions by π-A isotherm and UV-vis absorption measurements.The HD molecules formed H-aggregates in LB monolayers leading to a blue-shift of the absorption peak with respect to that of the monomeric HD in the chloroform solution and reduction of molecular hyperpolarizability β(the fundamental wavelength λ=1.064 μm)through a second-harmonic off-resonant effect. The H-aggregate formation mainly occurred and effective molecular hyperpolarizability β could be enhanced by raising the compressional speed and/or lowering the deposition surface pressure.