The present study developed a subsurface microhardness gradient technique to estimate hydrogen diffusivity of stainless steels, as per the similarity between concentration distribution and hardness gradient. Cathodic charging were performed on 304 stainless steels for 24 h in a 0.5 mol/L H2SO4 solution using a current density of 100 mA/cm2, with 0.25g/L Na2S as the hydrogen recombination poison. Microhardness in the cross-sectional region had an increase than the uncharged materials due to the hardening mechanism as found by martensite transformation. Hydrogen diffusivity was estimated using the microhardness data and the diffusion equation. The estimated diffusivity of hydrogen at 306 K in 304 stainless steels is 3.28×10-13 m2/s, which has good agreements with the one measured by time-lag electrochemical method in a previous research.