Brittle polylactide (PLA) was toughened by introducing 5 wt % of a poly(epsilon-caprolactone)(PCL) diol-and triol-based polyurethane (PU) network. The extent of crosslinking of the PU was varied by changing the ratio between diol and triol. The effects of the PU content and its crosslink density on the mechanical properties and the toughness of PU/PLA blends were investigated. Maximum toughness of PU/PLA blends, an order of magnitute higher than that of pure PLA, could be achieved by the use of a proper amount of PU and a proper extent of cross-linking.