A recent trend in the integration of electric power devices has caused problems of heat management because of high heat generation from the device. Therefore, studies of new materials having high thermal conductivity have been motivated. Polymer-based composites have been paid attention because of its flexibility and energy-saving productivity. In this study, nitride fillers, which have excellent thermal conductivity and electric insulating property, were used as a component of the polymer-based composites. To achieve high thermal diffusion with a small fraction of the fillers, the fillers were aligned by an electric field. Aluminum nitride (AlN) or boron nitride (BN) particles were dispersed in an acrylic polymer. An array of wire electrodes was used to form widely spread electric field. In-plane and through-plane alignment were carried out with varying arrangements of the electrode arrays. Thermal diffusivity in-plane direction of the AlN and BN composites increased about 1.2- and 1.7-fold by aligning the fillers as comparing with those without the alignment, respectively. To align the filler in through-plane detection, several kinds of the electrode arrangements were examined. As the best result, the through-plane thermal diffusivity of the aligned AlN composites became 30-fold compared to that without the alignment.