Summary: In this work the effects of the structure evolution of neat PLLA and its multi-wall carbon nanotube nanocomposites at an aging temperature of 45 °C (Tg-12 °C) have been investigated. The analysis of enthalpy relaxation rate (βH) of composites by means of differential scanning calorimetry (DSC) reveals the existence of a critical concentration of 1.25–2.5 wt. % in which aging rate reaches a mi–imum value of 1.351 J/g. Mechanical tests showed that neat polymer is much more sensitive to the aging process than the reinforced one. As shown by transmission electron microscopy (TEM) all the samples present randomly dispersed nanotubes, yielding an agglomerated structure at MWCNT contents exceeding 1.25–2.5 wt.%. The obtained results are explained from the viewpoint of polymer/filler interfacial effects that perturb the structural relaxation of glassy polymer due to the drastic reduction on the molecular mobility of polymer chains in the vicinity of carbon nanotube surfaces. In this way, nanocomposite approach represents an efficient strategy to successfully reduce the physical aging process of polymers.