Recent research revealed potentials to develop polymer matrix composite foams filled with thermally conductive filler network as light-weight thermal management materials. Since polymeric foams are commonly used for thermal insulation, the concept of thermally conductive polymer matrix composite foams seems to be counter-intuitive, and the underlying factors that govern polymer matrix composite foam's effective thermal conductivity (k eff ) were not clear. In this context, this paper develops new models to predict polymer matrix composite foams' k eff and to elucidate the dependence of k eff on their cellular morphology. Linear low density polyethylene-hexagonal boron nitride composite foams were used as case examples to verify the model. The model demonstrated that the composite foam's k eff would be promoted when the volume expansion was over a threshold percentage. At low hexagonal boron nitride loadings (e.g. 10 vol.%) and fixed cell size, linear low density polyethylene-hexagonal boron nitride foams' k eff increased with volume expansion percent through an increase in cell population density. Constrained foaming with preferential expansion in the heat flow direction also enhanced their k eff .