Poly(lactic acid) (PLA) and its blends with starch and methylenediphenyl diisocyanate (MDI) were extruded in a twin-screw extruder and compress-molded in a dog-bone shaped tensile bars to form test specimens. The effects of starch types (rice, corn, wheat, and potato) on mechanical properties of the PLA/starch/MDI composites were evaluated. Starch type and granule size had no significant effects on mechanical properties of the composite. But crystallinity of the composite decreased as the starch granule size increased. Potato starch was used to study effects of granule size (19 to 63 mu m) on mechanical properties of the PLA/starch composites. Both tensile strength and elongation of the PLAIstarchlMDI (55/45/0.5) composites reduced slightly as starch granule size increased from 19 to 63 mu m. While crystallinity of the composites was significantly decreased as granule size increased. Modulus of the composites remained the same regardless starch types and granule size.