The microstructure and mechanical properties of microcellular injection molded polyamide-6 (PA6) nanocomposites were studied. Cell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, were based on the material system and molding conditions. The correlation between cell density and cell size of the materials studied followed an exponential relationship. Supercritical fluid (SCF) facilitated the intercalation and exfoliation of nanoclays in the microcellular injection molding process. The orientation of nanoclays near the surface of microcells and between microcells was examined and a preferential orientation around the microcells was observed. Nanoclays in the microcellular injection molding process promoted the gamma-form and suppressed the a-form crystalline structure of PA6. Both nanoclays and SCF lowered the crystallinity of the parts. Microcells improved the normalized toughness of the nanocomposites. Both microcells and nanoclay had a significant influence on the mechanical properties of parts depending on the molding conditions. (c) 2005 Elsevier Ltd. All rights reserved.