Surface-enhanced second-harmonic generation (SHG) of the reflection from a surface covered with a Langmuir-Blodgett mono-molecular layer of fatty acid is studied under condition of surface plasmon excitation. Experimental results of SHG signals are in good agreement with theoretical calculations and the nonlinear polarizability coefficient d33 for a single arachidic molecule is estimated to be 0.32E-29 esu.