In order to study the effect of grain boundary on stress corrosion cracking (SCC) of α-brass, SCC tests were carried out on Cu-30.6 at%Zn alloy bicrystals in ammoniacal vapor at 303±2K. The susceptibility of the grain boundary to SCC was found to be higher than that of slip step sites in the present bicrystals with an about 15° off-coincidence Σ=13b boundary surface energy although it has been regarded to be as high as that of the so-called general boundary. Tubular pits which are considered to be initiated by the mechanism proposed by Pickering and Swann were mostly observed at the grain boundary and rarely on the crystal surface. Fine cracks propagating along the trace of {110} or {111} in the grain interior were observed not to grow into the main crack responsible to fracture.