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Magnesium matrix composites reinforced with short carbon fiber (SCF) (SCF/AZ31B composites) were fabricated by multi-times hot-extrusion with mass fraction of 3%. The effect of extrusion times and addition of SCFs on microstructure, mechanical properties of the composites were investigated. The results show that the size of grain was initially decreased while increasing the extrusion times, and SCFs were dispersed uniformly in the composites after quadruple-extrusion. The yield strength (YS) and ultimate tensile strength (UTS) of the quadruple-extruded SCF/AZ31B composites were increased by 50.15% and 63.60%, respectively, compared with the extruded AZ31B magnesium alloy. The fracture mechanism of SCF/AZ31B composite is dominated by brittle fracture. In addition, the grain refinement is attributed to dynamic recrystallization during plastic deformation and dislocations motion inhibited by SCFs. Multi-times hot-extrusion is an effective synthesis method to fabricate SCF reinforced Mg-based composites. Keywords: Short carbon fibers, Mg-matrix composite, Multi-times hot-extrusion, Dynamic recrystallization, Dislocation