Supertough polylactide (PLA)/polyurethane (PU) blends were prepared by reactive blending of PLA with polyester polyol and toluene‐2,4‐diisocyanate. The free volume and interfacial compatibility between the two polymers were investigated by positron annihilation lifetime spectroscopy. The PU particles dispersed homogeneously in the PLA matrix and self‐assembled into a subinclusion microstructure, resulting in fibrils and significant plastic deformation occurs during impact process. More phase interface and free volume cavities formed between PLA and PU boundary because of a good interfacial compatibility between the two polymers, leading to smaller sizes of free volume cavities, and the increasing of the number of these cavities. Therefore, the toughness of PLA was greatly improved by blending PU. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46596.