It is well documented that the strain rate sensitivity(m)increases at refined grain size for face-centered cubic(FCC)metals and alloys.Through a series of nanoindentation testing,however,we experimentally demonstrated a striking departure from conventional FCC metals that CoCrFeMnNi high entropy alloy(HEA)with FCC lattice structure exhibits monotonously decreased m as grain size reduced from～30.3μm to 7.2 nm.Moreover,the apparent activation volume v*,which generally shows an opposite trend of m,exhibited the identical decreasing trend with reduced grain size as that of m.Such an unusual trend of m and its correlation with v* in the FCC HEA alloys can be understood by a distinct deformation-mechanism-transitions and unique dislocation morphology evolution that differs from conventional FCC metals.