The fracture mechanism of polylactic acid (PLA) resin and the improving mechanism of its toughness by addition of an acrylic modifier were examined. Plane strain compression testing of PLA clearly showed strong softening after yielding. Because the stress for craze nucleation of PLA was close to the yield stress, brittle fractures resulted. The addition of an acrylic modifier to the PLA significantly lowered the yield stress and formed many voids. The release of the strain constraint because of the formation of many voids and the decrease of yield stress resulted in. the relaxation of stress concentration, and the toughness was improved. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1454-1460, 2010