Jute/poly(lactic acid) (PLA) composites were firstly prepared by extrusion and injection molding. Then the composites were exposed to a fungi environment for 28 d which consisted with five fungi according to the American Society of Testing Materials (ASTM) standards. Weight loss, morphology and mechanical properties of the samples with different infected days were measured. Gel permeation chromatography (GPC), scanning electron microscope (SEM) and differential scanning calorimetry (DSC) were utilized to analyze the infected process. Mechanism can be concluded that PLA on the surface of composites slowly degraded under the fungi environment. Then surface fiber were exposed in the air, the interface gap between fiber and matrix enlarged, resulting in the decline of the mechanical properties.