The dielectric properties of the nematic mesophase, p-methoxy benzylidene p-decyl aniline (MBDA), measured in planar geometry with a function of frequency and temperature are investigated in detail. The complex dielectric permittivity (ε′ and ε″) is also studied at a bias voltage of 10 V for planar aligned sample cell of nematic mesophase. The dielectric permittivity with bias voltage attains a higher (> 2 times) value than that without bias voltage at a temperature of 56 °C, which is due to the fact that the linking group of nematic molecules is internally interacted with an applied bias voltage. This is supported by observing an enhanced dielectric permittivity of nematic liquid crystal (LC) in the presence of bias voltage, which can be fully explained as the increasing of the corresponding dipole moment. The dielectric relaxation behaviors of nematic LC are also demonstrated for planar aligned sample cell. The remarkable results are observed that the relaxation frequency shifts into low frequency region with the increase of the bias voltage applied to the planar aligned sample cells. The dielectric relaxation spectra are fitted by Cole–Cole nonlinear curve fitting for nematic mesophase in order to determine the dielectric strength.