Using Maxwell displacement current (MDC) and second harmonic generation (SHG) measurements, the linear and nonlinear dielectric properties of organic monolayer films on a material surface can be examined. In this study, we have developed a novel experimental system which can detect MDC and SHG signals, and examined the molecular motion of 4-cyano-4 ' -n-alkyl-biphenyls (5CB) on the water surface by monolayer compression. The orientational order parameters expressed by using Legendre polynomials were successfully determined from both MDC and SHG signals, and the difference in the determined order parameters was discussed in association with the molecular motion of 5CB monolayers.