We　achieved　ultra-high　coercivity　of　5.65　kOe　in　L1(0)-Mn1.33Ga　alloy　via　the　hot　deformation　(HD)　method　and　revealed　the　hardening　mechanism.　Hot　deformation　led　to　recrystallization　and　grain　refinement,　where　the　average　grain　size　was　reduced　to　about　1.5　mu　m　for　HD-88%　magnet.　The　coercivity　mechanism　indicated　a　weak　pinning　model　by　magnetic　mini-loop　analysis.　It　was　found　that　small　grains　were　formed,　accompanied　by　a　certain　number　of　{111}　＜　11-2　＞　twins　for　HD　magnets.　The　magnetic　domain　observation　showed　that　both　the　sub-micro　grain　boundary　and　the　twin　boundary　were　acted　as　the　pinning　center　of　the　domain　wall,　but　the　latter　had　weaker　pinning　effect.　The　ultra-high　coercivity　of　the　HD-88%　Mn1.33Ga　magnet　originated　mainly　from　the　pinning　of　the　grain　boundary　enhanced　by　grain　refinement.　Further　grain　refinement　and　twin　structure　inhibition　will　be　promising　approaches　in　order　to　obtain　higher　magnetic　properties　for　L1(0)-MnxGa　bulk　magnets.　Published　under　an　exclusive　license　by　AIP　Publishing.