Blends of soy protein (SP) and a semicrystalline polylactide (PLA) were prepared using a twin-screw extruder. The melt rheology, phase morphology, mechanical properties, water resistance, and thermal and dynamic mechanical properties were investigated on specimens prepared by injection molding of these blends. The melt flowability of soy-based plastics was improved through blending with PLA. Scanning electron microscopy revealed that a cocontinuous phase structure existed in the blends with soy protein concentrate (SPC) to PLA ratios ranging from 30:70 to 70:30. SPC/PLA blends showed fine co-continuous phase structures, while soy protein isolate (SPI)/ PLA blends presented severe phase coarsening. At the same SP to PLA ratios, SPC/PLA blends demonstrated a higher tensile strength than SPI/PLA blends. The water absorption of soy plastics was greatly reduced by blending with PLA. The compatibility was improved by adding 1-5 phr poly(2-ethyl-2-oxazoline) (PEOX) in the blends, and the resulting blends showed an obvious increase in tensile strength and a reduction in water absorption for SPI/PLA blends. The compatibility between SP and PLA was evaluated by mechanical testing, dynamic mechanical analysis (DMA), water absorption, and scanning electron microscopy (SEM) experiments. Differential scanning calorimetry (DSC) revealed that PLA in the blends was mostly amorphous in the injection molded articles, and SP accelerated the cold crystallization and could increase the final crystallinity of PLA in the blends.