The foaming behavior and thermal properties of neat polylactide (PLA) and PLA/cotton-fiber composite foams were investigated in this study. CO2 saturation pressure, temperature, and fibers content significantly affected PLA's crystallinity and foaming behaviors. At the same saturation temperature (140 degrees C), a low CO2 pressure generated nonuniform foam and a large unfoamed area due to too high crystallinity with a close-packed structure. At an intermediate pressure, a fine-cell structure was developed due to the presence of numerous less closely packed crystals served as cell nucleating agents. A high CO2 pressure also led to a uniform cell structure but with larger cell sizes due to cell deterioration. Similar to the effect of saturation pressure, an intermediate temperature generated the uniform fine-cell structures. The PLA's cell morphology was improved by the addition of cotton fibers at a low content because of the increased local stresses through the fibers and transcrystals surrounding the fibers.