Samples of ultrafine-grained 304L stainless steel with an average grain size of (130±30) nm wereprepared by using equal channel angular pressing (ECAP) technique after 6 passes with route Bc at 500 ℃.The electrochemical behaviors and the passive properties of the material in 0.05 mol/L H₂SO₄+0.25 mol/L Na₂SO₄solution were examined by using polarization curves and Mott-Schottky analysis. As compared with the coarse-grained counterpart, the ultrafine-grained 304L stainless steel sample exhibits a lower corrosion potential(-505 mV) and a higher corrosion current density (11.7μA/cm²), indicating the accelerated trend of the activedissolution, and has a broadening of passive region (-341-847 mV) with lower passive potential (-341 mV) andpassivation current density (4.5μA/cm²). Both of the passive films exhibit n-type semi-conductor behaviorsat potentials ranging from 0 to 0.7 V. The ultrafine grains of 304L stainless steel are helpful in forming more stablepassive films with the reduced donor density and the diffusion coefficient of the charge carrier.