High-entropy alloys (HEAs) are a new class of materials with a potential engineering application,but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge.Here,we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing (FSP).After FSP,the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated,and the cast coarse two-phase structure (several micrometers) was changed into an ultrafine-grained single-phase structure (150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins.This unique microstructure was mainly attributed to the severe plastic deformation during FSP,and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs.Furthermore,FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa,respectively,which were ＞ 1.5 times higher than those of the base material.The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins.This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.