Abstract: The nanocrystalline Al0.3CoCrFeNi high-entropy alloy (HEA) with an average grain size of 30 nm was prepared by the high-pressure torsion (HPT) method. The microstructures and electrochemical properties in NaOH solution of the nanocrystalline HEA were investigated by electron backscatter diffraction, transmission electron microscope, potentiodynamic polarization curves and electrochemical impedance spectroscope and compared with those of the as-cast coarse-grained HEA. The results show that the microstructures of both coarse-grained and nanocrystalline HEAs exhibited a face-centered cubic structure, but the dislocation density of the nanocrystalline HEA was improved by 10 orders of magnitude than that of the coarse-grained HEA. Comparing to that of the coarse-grained HEA, the corrosion current density and passive current density of the nanocrystalline HEA were reduced by 42.9% and 21.6% respectively, showing the superior corrosion resistance, which was mainly ascribed to the high densities of grain boundaries and dislocations induced by HPT. Refining the grain size to nanoscale by HPT was an effective access to improving the alkali resistance of the high-entropy alloy.