The thermal conductivity (TC) of an isotropic composite comprising of a main-chain smectic liquid crystalline PB-10 polyester and 50-μm-sized roughly spherical magnesium oxide (MgO) particles is investigated. The increase in the composite TC with higher MgO fractions is steeper than that expected by Bruggeman's theory for the TC of a polydomain PB-10 polyester (0.52 W m−1 K−1). When the filler content is larger than 30 vol %, the composite TC approaches a value that can be explained only if the polyester functions as a matrix with 1.0 W m−1 K−1, which is five times as high as those of isotropic common polymers (0.2 W m−1 K−1). Such an unusually high TC for a polymer matrix is attributed to some polymer lamellae that lie parallel to the particle surface and are stacked toward neighboring particles, thus creating effective heat paths between the particles and a continuous thermal network in a composite. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39896.