The objective of this study was to explore the degree of improvement in the resistance of biodegradable polylactide (PLA) structure against atmospheric weathering (outdoor) conditions when reinforced with only 15 wt% E-glass fibers (GF). For this purpose, both neat PLA and PLA/GF composite specimens were exposed to accelerated weathering conditions of both UV-irradiation and moisture cycles in accordance with ISO 4892-3 standards for various periods until 400 h. Many characterization techniques used revealed that the alterations in the structure and properties of the specimens were due to the drastic decrease in the molecular weight of the PLA matrices via chain scission reactions. It was observed that reductions in the mechanical properties (strengthmodulus- toughness) of the neat PLA were much more critical compared to the reductions in the PLA/GF composite. For instance, the reduction in the tensile strength of the neat PLA specimen was as much as 92%; while that reduction for the PLA/GF specimen was only 34%. Because, inorganic strong glass structure of the GF reinforcements having almost no chemical degradation during weathering periods kept their actions in the composite strengthening-stiffening-toughening mechanisms.