Abstract: Microstructure of Mg-4Al-2Si(AS42) magnesium alloy was refined by reciprocating extrusion(RE) process. Refinement effect, breaking feature, regularity of distribution and breaking mechanism of Mg2Si particles were investigated. The results show that the particles dimension decreased constantly, the eveness and dispersity of Mg2Si particles increased with increasing the passes of RE from as-cast to 8 passes, however the particles coarsened at RE-11 passes. The peak value of compression stress was about 75 MPa, which was greater than the fracture stress (48 MPa) of Mg2Si particles at 400 ℃ and strain rate of 0.01 s-1, resulting in the particles fractured after RE. The density and amount of the dislocation which assembled around Mg2Si particles increased with increasing the RE passes, and the stress concentration increased rapidly that led to the particles breaking.