Molecular monolayer at the air-water interface was investigated using second-order nonlinear optical processes (surface sum-frequency generation and second-harmonic generation). Laser outputs at different frequencies could selectively excite and probe the desired part of the chemical bonds or sub-moietiesof a model molecule, pentylcyanoterphenyl. By using different polarization combinations of input and output beams in the measurement, all the independent nonlinear optical susceptibilities of each part could be deduced. It was determined the orientation of cyano group and terphenyl ring was about 50。 from the surface normal, and that of the terminal CH₃ group was 58。, with the pentyl chains in the layer mostly straight conformation. Thus a quantiative and self-consistent picture of the molecular conformation at the interface was obtained.