The　mechanical　properties　of　traditional　dispersion　-strengthened　composites　are　greatly　limited　because　of　the　weak　interfacial　bonding　between　the　metal　matrix　and　ex　situ　reinforcement.　Here　we　report　a　copper　matrix　composite　with　high　hardness　strengthened　by　high-density　dispersed　nanoscale　in　situ　boron,　having　great　potential　in　structural　and　functional　materials,　such　as　wear-resistant　bearings.　The　hardness　of　the　composite　can　reach　3.04　GPa　by　a　nanoindentation　test.　The　composite　was　fabricated　by　a　powder　metallurgical　process,　in　which　the　boron　was　produced　in　a　copper　melt　directly.　Nanotwins　and　lattice　distortion　zones　observed　in　the　phase　boundaries　may　contribute　to　the　ultrahigh　hardness.