The second order non-linear polarizability of Langmuir-Blodgett multilayer formed with one sort of molecules is zero because of the counteraction of non-linear polarization in adjacent monolayers which have opposite orientation. The magnitude and sign of effective non-linear optical coefficient of Langmulr-Blodgett monolayer with different polar groups have been studied experimentally by optical second harmonic gener ation technique. We alternatively assembled the monolayers which have opposite sign of polarizability and successfully have their non-linear polarizability superposed construtively. In this way, we can obtain a molecular membrauce poseessiag much larger effective second order non-linear optical coefficient.