The work aims to improve the mechanical property of polylactic acid (PLA) by adding an appropriate amount of coir fiber (CF), so as to adapt to the product packaging. Coir fibers were pretreated by alkali treatment, and then mixed with PLA to make coir fiber-reinforced PLA composites (CF/PLA) with melt-blending method. The tensile strength, impact strength, micro-morphology of surface and the dynamic thermomechanical property of composites at different CF content were investigated. Compared to pure PLA, the impact strength and the storage modulus of composites increased, while the tensile strength and glass transition temperature decreased. At alkali-treated coir fiber content of 3%, the value of impact strength of composites was increased by about 24% as compared to that of neat PLA. Thus CF/PLA blends reinforced with the treated fibers exhibited superior mechanical properties than neat PLA.