Blends of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) with various contents (0, 5, 10, 15, 20 and 30 weight %) and with different molecular weights (M̅w = 1000, 4000 and 6000 g/mol), called respectively PEG1, PEG2, and PEG3 were prepared by melt blending. Since glass transition temperature (Tg), Tα and loss factor (tan δ) are relevant indicators of polymer chain mobility, plasticisation has been studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Low molecular weight PEG enable increased miscibility with PLA and more efficient reduction of glass transition temperature (Tg) for concentrations of PEG less than 20%. This effect is not only enhanced by the low molecular weight but also by increasing its content up to 20%. As expected, both Tα and Tg decrease when increasing PEG molar mass and content up to 20%, which demonstrates the effectiveness of PEG to act as a plasticizer of PLA.The plasticisation of PLA with PEG effectively lowers Tg and cold crystallization temperatures. The use of low molecular weight PEG decreases the intermolecular forces and increases the mobility of the polymer chains, by improving the flexibility of PLA. The effect of the use of various molecular weights of PEG is confirmed by thermogravimetric analysis (TGA).