Sulfide stress cracking (SSC) occurs in steel exposed to sour environments. Ni promotes the formation of a crack-like corrosion morphology (so-called stress groove or fissure), which accelerates the SSC; however, the mechanisms underlying fissure formation and growth are not fully understood. In this study, the effects of Ni on SSC and fissures were investigated. Constant-load tests were conducted at various potentials using TS 800 MPa grade steel with and without 1 % Ni addition. The fissures were formed near the corrosion potential on adding Ni. Anodic or cathodic polarization promoted SSC, suggesting that SSC was affected by both active path corrosion (APC) and hydrogen embrittlement (HE) mechanisms. In addition, the fissure shapes and crystal orientations of the microstructure were analyzed to classify the fissures as dissolution (corrosion) or mechanical cracking. Further, characteristic grooves promoted by Ni were confirmed to be APC.