Composites composed entirely of biodegradable polymer materials such as polylactic acid (PLA) and natural fibers are attracting attention as alternatives to petroleum-based plastic materials. Since fibers have low dispersibility in bulk resins such as PLA, the use of fiber surface treatment agents can improve the fiber dispersibility and the physical properties of the composites. In this study, accelerated degradation tests were conducted at 60°C for upto 2000 h on specimens containing cotton fibers at a ratio of 40 wt% to the PLA matrix. The results showed that the interfacial adhesion between the PLA matrix and cotton fibers could be improved by coating the cotton fibers with a reactive surface treatment agent. Furthermore, the addition of 5 wt% of epoxidized linseed oil, a medium-molecular-weight reactive surface treatment agent, not only promoted the crystallization of the PLA/cotton-fiber composites at 60°C, but also improved the thermal and mechanical properties with the degradation time.