The Mg matrix composite (Mg2Si/Mg-5Zn-2.5Er) was prepared using repeated plastic working (RPW) technique. and the effects of the number of RPW cycles on the microstructure and mechanical properties of these composites were investigated. The results indicated that the added silicon particles fully reacted with the magnesium matrix, and theMg2Si/Mg-5Zn-2.5Er composites were successfully achieved. When the number of RPW cycle increased, the size of the Mg2Si particles decreased, and the grain size of the matrix alloy reached the minimum when 200 RPW cycles was used. The best mechanical properties were also identified as 394 MPa ultimate strength, and 363 MPa yield strength, when 200 RPW cycles were used.