The formula of dielectric polarization of Langmuir monolayers with C-infinitynu-symmetry has been derived, using orientational order parameters expressed by Legendre polynomials. It is shown that Maxwell Displacement Current (MDC) measurement coupled with Optical Second Harmonic Generation (SHG) measurement is helpful for the study of orientational orders and the phase transitions in monolayers. Using this measurement, monolayers of 4'-n-octyl-4-cyanobiphenyl (8CB) on the water surface was examined by monolayer compression. The phase transition from planar to polar orientational alignment phase was clearly predicted in the region of low surface pressure.