Polymers incorporated with high thermal conductivity fillers have numerous applications in thermal interface materials. Plenty of efforts have been made to improve the thermal conductivity of polymer composite. A possible method is to choose fillers with different morphologies, which can combine the advantages of various fillers. However, owing to the limitations of the effective medium theory as well as lack of researches of thermal percolation, there is still little understanding of the synergistic mechanism of fillers with different morphologies. In order to avoid the coupling effect of different materials, this work uses the same kind of Al₂O₃ but with different morphologies to prepare different kinds of epoxy composites incorporated with spherical Al₂O₃, plate-like Al₂O₃ and fillers mixed of 1∶1 ratio. The thermal conductivity of each sample is measured by the steady state method. With the fitting of the thermal percolation theory, the synergistic effect of plate-like fillers and that of spherical fillers are verified to promote the formation of thermal percolation network. In addition, by observing the microscopic distribution of fillers, we try to explain the mechanism of this synergistic effect.