Magnesium　matrix　composite　reinforced　by　titanium　alloy　(Ti-6Al-4V)　particulate　was　fabricated　by　powder　metallurgy　route.　The　samples　of　composite　were　hot-extruded　with　an　ultra-deformation　ratio　of　225:1.　The　microstructures　of　extruded　samples　were　investigated　using　optical　microscopy,　scanning　electron　microscopy　(SEM)　and　transmission　electron　microscopy　(TEM).　The　results　showed　that　the　Ti-6Al-4V　particulates　were　apparently　elongated　along　the　extrusion　direction,　thus　increased　its　reinforcing　effect　on　the　matrix.　The　ultra-deformation　could　markedly　decrease　the　grain　sizes　of　the　composite　matrix,　and　greatly　improve　the　homogeneity　of　microstructures　when　compared　with　a　lower　ratio　of　extrusion　deformation;　in　addition,　the　oxide　layers　on　the　surface　of　original　Mg　particles　were　effectively　broken　and　dispersed　randomly　in　the　vicinity　of　Mg-Mg　interfaces.　It　produced　an　oxide-dispersion-strengthening　(ODS)　effect,　further　reinforced　the　matrix　as　a　kind　of　secondary　reinforcement.