A　Co30Cr20Fe18Mn18Ni11Si3　high-entropy　alloy　(HEA)　with　low　stacking　fault　energy　and　high　strengthen　was　designed　based　on　the　principle　of　heterostructure　strengthening.　The　alloy　was　processed　into　a　multiscale　heterogeneous　microstructure　containing　hierarchical　twins.　The　alloy　achieved　an　ultrahigh　yield　strength　of　1500　MPa,　ultimate　tensile　strength　of　1750　MPa　and　a　large　ductility　of　20　%　at　77　K.　These　mechanical　properties　were　superior　to　those　of　most　FCC　HEAs　reported　in　the　literature,　breaking　the　strength-ductility　trade-off　of　conventional　metal　alloys.　Such　extraordinary　mechanical　properties　were　attributed　to　a　suitable　strain　hardening　capability,　stemming　from　the　synergistic　effect　of　hetero-deformation-induced　hardening,　twinning-induced　plasticity,　and　deformation-induced　phase　transformation　during　tensile　deformation.