Research　about　the　size　effect　had　been　the　major　subject　in　the　research　of　micro　forming　for　the　past　few　years.　By　now,　the　strain　gradient　theory　had　been　used　well　in　explaining　the　size　effect,　but　there　was　little　research　about　the　metal　micro-forming　basing　on　the　strain　gradient　theory.　Further　research　about　the　strain　gradient　hardening　on　the　flow　stress　during　the　thin　metal　wire　torsion　was　conducted.　And　a　new　user　defined　subroutine　considering　the　strain　gradient　hardening　was　coded　basing　on　the　finite　element　software:　Abaqus.　Using　the　subroutine,　the　simulation　of　the　thin　copper　wire　torsion　experiment　was　conducted　to　study　the　influence　of　the　strain　gradient　hardening　during　the　torsion.　Compared　with　the　experiment,　the　simulation　results　matched　well　with　the　experiment.