A theoretical expression of reflected and transmitted second harmonic generation (SHG) with different polarization combinations on a Langmuir Blodgett monolayer in front geometry and back geometry were given as a function of incident angle. Transmitted SHG signals were measured on a stilbazium salt LB monolayer with variable angle of incidence. It showed that the molecular second order polarizability β and molecular tilt angle 〈ψ〉 could be obtained by transmitted SHG measurement using only one polarization combination, provided that the variable angle measurement was made under a full 360 degree range. This a new method for obtaining molecular β and 〈ψ〉 in LB monolayer by SHG.