It is common to use fillers to increase the thermal conductivity of polymer materials. Filler conductivity and filler content have been identified as the dominating factors influencing the overall effective thermal conductivity of composite materials. The interactions between the fillers and the base materials have been seldom considered. This research addresses this problem by solving the heterogeneous two-dimensional conjugate heat conduction problem in the composite materials. The interactions between the fillers and the base materials are considered. It is found that the filler shapes and their orientation angles have great impacts on the effective thermal conductivity of the composite materials. Of the various shapes, the I shaped fillers are the best choice for composite polymer materials. The influences of filler shapes and angles explain the reason why there are large discrepancies between the measured data and the available correlations in literature, which were searched for many years for the accurate predictions of the effective thermal conductivity of composite materials. The slices fillers are effective only when they are distributed parallel to heat flow directions. Currently the I shaped and T shaped fillers are promising and they should be used for future materials fabrications. (C) 2013 Elsevier Ltd. All rights reserved.