The short carbon fiber (CFs) reinforced AZ91 Mg matrix composites with different CFs volume fractions (10vol%, 15vol%, 20vol%) were fabricated by stir-casting method. The cast CFs/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 CFs distribute parallel to the extrusion direction. With increasing of the extrusion temperature and extrusion ratio, the porosity ratio of the CFs/AZ91 composites is reduced effectively and the recrystalled grains grow up accompanied by the improvement of the microstructure homogeneity. The tensile properties of CFs/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 CFs/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. © 2019, Editorial Office of Acta Materiae Compositae Sinica. All right reserved.