Maxwell displacement cut-rent (MDC) measurement has been employed to study the dielectric property of Langmuir-films, A method for determining the dielectric relaxation time of floating monolayers on the water surface is presented. MDC flowing across the monolayers is analyzed using a rod-like molecular model. It is revealed that the dielectric relaxation time of monolayer in the isotropic polar orientational phase is determined using a linear relationship between the monolayer compression speed and the molecular area A(m). Compression speed alpha was about 30, 30, 50mm/min.