The goal of this study was to compare accelerated weathering performance of neat polylactide (PLA) and its 1 mass% organically modified montmorillonite (Mt) nanocomposite; compounded and shaped by twin-screw extrusion melt mixing and injection molding, respectively. Accelerated weathering test system applied consecutive steps of UV irradiation and humidity in accordance with ISO 4892-3 standards for 200 h. Chain scission reactions such as photolysis, photooxidation and hydrolysis resulted in significant decrease in the molecular weight of PIA; consequently reductions in the mechanical properties of modulus, strength, ductility and toughness of the specimens occurred. However, after comparing mechanical properties of PLA and MtPLA before and after 200 h accelerated weathering, use of PLA with only I mass% Mt was extremely beneficial not only for "indoor applications" but also for "outdoor applications". This was due to the effective nanoscale reinforcing and barrier actions of intercalated/exfoliated Mt layers. For example, flexural strength was 6% beneficial before weathering, but after weathering the benefit was 88%. (C) 2015 Elsevier B.V. All rights reserved.