The effect of the thickness on the mechanical properties of injection-molded specimens of pure polyamide-6 (PA6) and polyamide-6 clay nanocomposites (PA6-NC) with 5 wt% of layered silicates was investigated. Plates of 0.5, 0.75, 1 and 2 mm thickness were characterized in the injection direction using Dynamic Mechanical Analysis under torsion and tension respectively, and tensile tests. The fracture surfaces were analyzed by Scanning Electron Microscopy. In contrast with PA6, PA6-NC showed thickness effect and clear differences in the mechanical and thermomechanical properties between skin and core, especially in the 2 mm thick samples. Increasing thickness in PA6-NC led to a reduction of tensile modulus and yield stress. In the fracture surface of the thicker tensile specimens the formation of a sheet-like structure was observed. Multiple voiding in the core causing initial failure in this region and a stiffer skin with a better orientation of the layered silicates in the injection direction are two important elements of a micromechanical model proposed in this paper to explain the fracture mechanism in PA6-NC. (C) 2003 Elsevier Science Ltd. All rights reserved.