A spherical boron nitride with a card-house structure (sBNch) was developed to enhance the filler's thermal property. To develop a thermal conductive insulating composite material with higher thermal conductive and acceptable electrical breakdown strength, an epoxy (EP)/an sBNch composite was produced. The treeing breakdown voltage of the composite using a needle-rod electrode and the thermal conductivity were measured. The treeing breakdown voltage of EP/sBNch composite with the sBNch content of 50 wt% was higher than that of EP without the sBNch. The protrusion on the surface of the sBNch increases the effective distance between the electrodes. The thermal conductivity of EP/sBNch composite with the sBNch content of 50 wt% was higher than that of EP. From these results, the sBNch is considered a strong candidate filler of a thermally conductive composite. (c) 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.