Second- harmonic generation (SHG ) of stilbazium salt Langmir-Blodgett monolayer at different surface pressure was measured. We got the enhanced SHG signal at low surface pressure. Refering to surface pressure-area isotherm and the transparent spectra of stilbszium salt we got following results. The stilbazium salt on LB film was in the form of solid state. The absorption peak 351 nm of the solid aggregation at the low surface pressure led to resonant-enhancement of the fourth order nonlinear optical susecptibility Ｘ(4)(—2ω:ω,ω,ω,—ω) which gives an effecton to SHG signal.