Nylon 6-polyethersulfone (PES)-nylon 6 block copolymers were synthesized by anionic polymerization of E-caprolactam using chlorine-terminated PES, a polymeric activator. The structure and properties of these block polymers were examined using infra-red, nuclear magnetic resonance, differential scanning calorimetry, transmission electron microscopy, and thermogravimetric analysis. The compatibilizing effects of this block copolymer on PES/nylon 6 blends were investigated by examining thermal properties, morphology and dynamic mechanical characteristics. When the block copolymer was added, the size of dispersed phase in the blend decreased dramatically, and the glass transition temperatures of the constituent polymers converge. The improvement of dynamic tensile modulus and thermal resistance due to the dispersed PES phase were enhanced by the added block copolymer.