Hydrogen embrittlement (HE) and electrochemical corrosion behaviors in the Cu-IF steel and the high strength low alloy (HSLA) steel were investigated for meeting the general demand that an advanced material with excellent material properties should be developed and applied to actual components and structures. Experimental results revealed that a critical nanometer size of copper precipitation particles beyond which a good protective passive film could not be maintained did not exceed 2-4 nm equivalent to the size of copper clusters precipitated in the peak aged stage. Furthermore, the grown copper particles, such as epsilon-copper, were preferable to the copper clusters and/or the twinned 9R structures as trapping site for hydrogen and contributed the suppression of HE. On the other hand, regarding the HSLA steel, it was apparent that the susceptibility to HE and the apparent diffusion coefficient of hydrogen in the steel, D-H, strongly depended on applied cathodic potential. There was a unique correlation between both, and the susceptibility to HE increased monotonously with increasing D-H. Consequently, the change in entrance rate of hydrogen into the steel with applied cathodic potential seemed to be responsible for the potential dependence of susceptibility to HE.