This study describes the toughening of poly(lactic acid) (PLA) through blending it with poly(ethylene oxide)- poly(propylene oxide)- poly(ethylene oxide) triblock copolymers (EPE, Pluronic (R)) at loadings less than 10 wt %. The miscibility of EPE copolymers in PLA was examined by thermal, morphological, and rheological analysis. Copolymers with low molecular weight and low poly(propylene oxide) (PPO) content were more miscible with PLA but less efficient as a toughening agent. In contrast, copolymers with high PPO content and high molecular weight formed droplets in the PLA and significantly increased the ductility of PLA at 5 wt% loading. More importantly, it was found that the toughening mechanism in those toughened PLA/EPE blends is similar to "rubber toughening of brittle plastics" rather than plasticization. Since the refractive indices of PLA and EPE are nearly matched, the blends were transparent. For the first time, the rheological properties of PLA/EPE blends are studied. Both the miscible and immiscible blends exhibited lower viscosity than neat PLA on account of different mechanisms. The low viscosity of the miscible blends resulted from the plasticization effect of the molecularly dissolved EPEs while the low viscosity of the immiscible blends resulted from the lubrication effect of the phase-separated EPE liquids. This lubrication effect also led to low adhesion on metal surfaces.