Abstract: In this paper, a co-rotating twin screw extruder connected with a melt pump was employed to carry out the extrusion foaming of a semi-crystalline PLA. The effect of CO2 concentration on PLA foam density, cell density, and cell diameter was investigated. With 7 % to 10 % CO2, PLA foam with low density of 40 kg/m3，small cell diameter of 285 μm, and high cell density of 1.5×106/cm3 was obtained. With increasing CO2 content, the initial crystallization peak of PLA foam decreased and the degree of crystallization increased, indicating that the supercritical CO2 promoted the crystallization of PLA in the extrusion foaming process.