Plastics or polymers of high thermal conductivity are highly desired in various industries. Adding fillers of high thermal conductivity to the base materials is a solution to make composite plastics of high thermal conductivity. Previous researches were focused on increasing the thermal conductivity of the composite materials by increasing the filler content and the thermal conductivity of the fillers. Relatively little attention was paid to the optimization of filler shapes. In this study, the effects of the filler shapes on the thermal conductivity of the composite materials are investigated, where the filler shapes are artificially designed. Heat conduction between the base materials and the artificially designed fillers is modeled. It is found that the filler shapes have great impacts on the effective thermal conductivity of the composite materials. Of the various shapes, the double Y shaped fillers are found to be the best choice for composite materials in which the fillers are distributed randomly. In future industrial applications, new filler shapes, such as double Y, Y, quad Y shaped, I and T shapes should be specially produced to replace the traditional fillers shapes: particles, fibers or slices. At last, composite materials made of paraffin wax and steel fillers of ten shapes are fabricated to simulate and validate the results. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39550.