Abstract: The nucleation and propagation of stress corrosion cracking（SCC） and hydrogen-induced delayed cracking（HIDC） in various hydrogenant environments, such as H2S and H2 gases, water, aqueous solution of H2S and charging were followed metallographically, using polished WOL type constant deflection specimen. The results of investigation show that if KI>KI cc and the strength of steel is larger than a critical value, the size of the plastic zone and the degree of plastic deformation ahead of the crack is enlarged continuously with time, i. e. a hydrogen-induced delayed plasticity（HIDP） is observed and SCC and HIDC then nucleate and propagate when HIDP develops to a critical condition. For charging specimen, HIDC nucleated preferentially on the surface of a specimen, then HIDP on the surface of the specimen was not caused by pre-produced crack in the interior of the specimen.The effect of hydrogen on the apparent yield strength of a smooth tensile and precrack twist specimen are insignificant. However, for the bending or I and I -III complex precrack specimen with a tensile stress gradient, hydrogen decreases the apparent yield stress considerably if the strength of steel and amount of hydrogen exceed critical values. This is the cause of HIDP and then HIDC. The variation of the KI sec and da/dt with the strength of steel and the environments has been explained.The effect of loading rate, test temperature and pre-plastic deformation on hydrogen induced apparent yield stress reduction have been investigated. According to these results, a new mechanism of SCC or HIDC in low-alloy steels has been discussed.