Abstract: The short carbon fiber (CF_s) reinforced AZ91 Mg matrix composites with different CF_s volume fractions (10vol%, 15vol%, 20vol%) were fabricated by stir-casting method. The cast CF_s/AZ91 composites were extruded into bars with three different extrusion ratios at 250℃ and 350℃, respectively. During the hot extrusion, grains of the AZ91 matrix are refined due to the dynamic recrystallizaiton while the CF_s distribute parallel to the extrusion direction. With increasing of the extrusion temperature and extrusion ratio, the porosity ratio of the CF_s/AZ91 composites is reduced effectively and the recrystalled grains grow up accompanied by the improvement of the microstructure homogeneity. The tensile properties of CF_s/AZ91 composites are improved due to the improved microstructure whereas the fiber fractures obviously during extrusion, which will restrict the further improvement of the mechanical properties. The CF_s/AZ91 composite extruded with small extrusion ratio at lower temperature has better damping capabilities than those of the composites extruded with high extrusion ratio and higher temperature.