Polymer composites containing boron-based materials are well-known absorbers of thermal neutrons, and thus may be used as radiation shielding materials. High-density polyethylene (HDPE) composites with chemically modified boron nitride (mBN) fillers, functionalized with an organosilane, were manufactured by twin screw reactive extrusion(REX) process. The structure and physical properties of these HDPE/BN composites were compared with those of the composites containing pristine BN and boron carbide (B4C) fillers. In HDPE/BN composites, many agglomerated pristine BN particles were observed with a broad size-distribution ranging from 1 to 8ìm. A plenty of interfacial voids, which could be formed during fracture, were observed at the interface between the HDPE and BN. On the other hand, the SEM micrograph of the composite containing mBN exhibited a good dispersion of particles in the matrix with a small particle size and narrow size distribution of 1–4ìm and few interfacial voids as shown in Figure 1. As a result, the HDPE/mBN composites showed a better dispersion state of the filler particles, higher tensile modulus and greater effective thermal conductivity and superior neutron shielding property than HDPE/BN and HDPE/B4Ccomposites. In addition, the morphology and the better neutron shielding properties were compared with the HDPE/BN and HDPE/B4C composites as shown in Figure 2. This work demonstrates that for the preparation of high-performance radiation-shielding composite materials, an appropriate interfacial adhesion property between polymer and fillers is crucial and should be carefully controlled.