Although there is considerable research effort in the development of halogen-free flame-retardant polylactide (PLA), a bio-based aliphatic polyester, the potential synergistic interactions of nanoparticles and conventional additives have scarcely been considered. Therefore, this study aimed to investigate the influence of nanofiller geometry on fire retardancy of PLA nanocomposites containing aluminium diethylphosphinate (AlPi) flame retardant. For this purpose, a variety of nanofiller geometries were utilized, spherical nanosilica (0-D), rod-like halloysite (1-D) and plate-like montmorillonite (2-D). Fire performance increased in the order of rod-like < spherical << plate-like geometries which matched qualitatively with the effective surface area of nanoparticles in the nanocomposite. Owing to intercalation/exfoliation (nanodispersion) of montmorillonite, a large surface area was obtained which facilitated rapid migration and accumulation of platelets on exposed sample surface prior to intumescent char formation. By this means, intumescence was hindered and a thicker aluminium phosphate/montmorillonite nanocomposite char was formed acting as a strong transport barrier. Investigations on microstructural evolution of residues revealed extraordinarily regular, hollow-fibrillar structures during combustion of PLA/AlPi. The corresponding nanocomposites formed micro-porous ceramic-like structures composed of spherical aluminium phosphate nanoparticles, covering the surface and protecting the pyrolysing melt. (C) 2012 Elsevier Ltd. All rights reserved.