Abstract: The poly(lactic acid) (PLA)/wheat straw fiber (WSF) composites were prepared by a melt-blending process. The effects of coupling agent, γ-aminopropyltriethoxysilane (KH550), on the mechanical properties, moisture absorption and interface performance of the composites were investigated. Moreover, the chemical structure of WSF was characterized by FTIR, and the morphology of the composites was detected by SEM. The results indicated that the reaction between the WSF and KH550 reduced the surface polarity of WSF, and the composites exhibited a decreasing trend in mechanical properties with an increase of WSF content. The mechanical properties of the composites were improved due to the modification of WSF with KH550, and their water absorption was also reduced after the WSF were modified by KH550. The SEM observation suggested that the interfacial compatibility of PLA with WSF was enhanced due to the surface treatment of WSF with KH550.