The microstructure of a mesogen-containing epoxy resin that exhibits high thermal conductivity (0.33 W/m K) was investigated by observations of the scanning electron microscope (SEM) and the polarized optical microscope (POM). It was found that there are many spherulite structures formed in the resin. We supposed that the reason for the high heat-conductivity obtained in the resin is ascribed to the existence of the spherulite morphology where highly ordered lamellar structures aggregated. The argument was strongly supported by preparing the resin films with size-changed spherulites which are induced by changing the initial curing temperature. The result revealed that the thermal conductivity increased with the increase in spherulite size. On the other hand, we have developed the spherulite size in the resin from 10 to 80 mu m in diameter that resulted in a greatly improved thermal conductivity of up to 1.16 W/m K in the highest value. This study may provide a useful method to improve the thermal conductivities of mesogenic resins themselves by producing spherulite structures. (C) 2012 Elsevier Ltd. All rights reserved.