Polymerized soybean oil (polySOY) and isotactic poly(L-lactide) (PLLA) were melt blended to increase the toughness of PLLA in an all renewable blend. The polySOY samples were prepared by crosslinking soybean oil by the addition of a free radical crosslinking agent or by heating the oil in the presence of oxygen. Soybean oil is relatively nonreactive compared to other vegetable oils, and conjugation of the double bonds within the fatty acid chains of the soybean oil triglyceride prior to crosslinking led to significantly increased reactivity. Poly(isoprene-b-L-lactide) block copolymers were used to compatibilize the blends due to the high degree of immiscibility between PLLA and polySOY. The blending of polySOY and PLLA resulted in significant improvements in the tensile toughness of the blend compared to neat PLLA. The blend morphology was dependent on the polySOY gel fraction or weight-average molar mass; the polySOY characteristics were key indicators of the tensile toughness.