The nonlinear optical properties of Langmuir-Blodgett films of four hemicyanine dyederivatives, which are similar in molecular structure, have been studied by optical secondharmonic generation (SHG)、 UV-Vis absorption spectrum and isotherms. The relation ofthe β to the molecular structure was investigated and the main reason resulting in the βdifferences was discussed as well. The sequence of polar chemical groups was proved. Thedifference of the degree of aggregation in LB films was analyzed. The quality of LB films was compared through the relation of optical SH signal intensity to the layer number of dye multialayers.