Effects of reciprocating extrusion on the microstructure and mechanical properties of the as-cast Mg-6%Si alloy were studied. Results show that the microstructures of the Mg-6%Si alloy are refined markedly after reciprocating extrusion with 4 passes; the coarse dendritic primary Mg2Si phase changes into fine granules, the size of the Chinese-character-like eutectic Mg2Si phase also decreases gradually and more uniform distribution of the granular Mg2Si phase in the matrix is obtained. After reciprocating extrusion with 4 passes, the tensile strength and the elongation are increased by 82.3% and 810.9%, respectively, compared with those of the as-cast alloy. Fracture analysis shows that the fracture mode of the alloy changes from brittle fracture to ductile-brittle fracture after reciprocating extrusion