Ultrasonic nanocrystal surface modification (UNSM) treatment on non-equiatomic medium-and high-entropy alloy (HEA) of Fex(CoCrMnNi)100-x is firstly introduced and its impact on microstructure and mechanical properties are revealed.By UNSM,severe plastic deformation-induced dislocation and defor-mation twins (DTs) arise at the topmost surface.Especially,Fe60(CoCrMnNi)40 (Fe60),which is classified as a medium-entropy alloy (MEA),exhibits ε-martensitic transformation.In the room temperature ten-sile test,a high strength of～600 MPa and ductility of～65 ％ elongation (strain to failure) is accomplished in Fe60.Initially formed DTs and ε-martensitic transformation by UNSM treatment plays a key role in retardation of necking point via both twinning-induced plasticity and transformation-induced plasticity.However,Fe20(CoCrMnNi)80 (Fe20) comparatively shows low strength of ～550 MPa and ～40 ％ elonga-tion,owing to the low accommodation of DTs than Fe60.Our research will provide new guidelines for enhancing the mechanical properties of MEA and HEA.