Structural and optical non-linear properties of four hemicyanine derivatives with similar molecular compositions but different donor-acceptor groups in Langmuir-Blodgett (LB) films have been studied by optical second-harmonic generation, UV-visible absorption and pi-A isotherm measurements. The magnitude of the molecular second-order polarizability beta in LB films was affected significantly by types of donor-acceptor group, and aggregation formations through a second-harmonic resonant effect. Selection of donor-acceptor also affects the molecular amphiphilicity and structural degree of order in LB films.