Magnesium matrix composite reinforced with SiC particle was fabricated by compocasting method. The SiCp/AZ91 composite was extruded initially, then subjected to ECAP. The microstructure and tensile properties of the composite at ambient and elevated temperature were investigated. After ECAP, the matrix alloy was significantly refined due to the dynamic recrystallization occurred during ECAP. Both the ambient yield stress and ultimate tensile stress of SiCp/AZ91 were increased after 1-pass and 2-pass ECAP and decreased after 4-pass ECAP. The ECAP processed composite exhibited superplasticity at elevated temperature, which is mainly due to the grain refinement of matrix alloy. The dominant deformation mechanism of the ECAPed composite at high temperature was grain boundary sliding. However, the sliding was prohibited due to the present of SiC particle.