This study aims to elucidate the nucleation processes of transgranular stress corrosion cracking occurring in aluminum alloy 6061-T651 immersed in NaCl solution. The specimen was subjected to constant loading in the solution, and corrosion phenomena were investigated from aspects of precipitates, the site and geometry of corrosion pits and cracks, and the crystal orientation where cracking occurred. It was found that corrosion pits were formed on a smooth surface, and then stress corrosion cracks tended to occur originating from deeper pits in which the stress/strain concentration was high, along the specific crystal plane {1 1 2}.