Abstract: Maleic anhydride (MAH) esterified starch was synthesized by dry method with corn starch and MAH as raw materials, and esterified starch/polylactic acid (PLA) composites were prepared by melt extrusion. Effects of MAH amounts on the degree of crystallinity and compatibility of composites were studied. At the same time, the influence of compatibility and degree of crystallinity change on thermal property, melt flow property, mechanical property, water resistance and rheology property of composites were investigated. FTIR results show that MAH esterified starches were successfully synthesized by dry method. Degree of substitution of esterified starch gradually increases with the increasing of MAH amounts, and the reaction efficiency is up to 90%. XRD and DSC results show that with the increasing of MAH amounts, the degree of crystallinity of composites decreases, which makes the compatibility between starch and PLA increase gradually. The decrease of degree of crystallinity and the improvement of compatibility cause the decreased glass transition temperature and the increased melt flow and water resistance. The mechanical properties and rheological properties are affected by the combination of the compatibility and degree of crystallinity. The tensile strength, bending strength, storage modulus and complex viscosity increase gradually when MAH amount increases from 0 to 1.0wt%, but these properties decrease gradually when the amount of MAH is more than 1.0wt%.