The effects of single-step thermomechanical treatments on the grain boundary character distribution (GBCD) and intergranular corrosion of Alloy 690 (Ni-30Cr-10Fe, wt.%) are investigated. High proportion of low Sigma CSL grain boundaries (more than 70% according to Palumbo-Aust criterion) associating with large size grains-cluster microstructure is obtained through one-step thermomechanical treatment of 5% cold rolling followed by annealing at 1,100 degrees C for 5 min. Nucleation density of recrystallization and multiple twinning are the key factors affecting the GBCD. The grains-cluster is produced by multiple twinning starting from a single recrystallization nucleus. That the mean size of the grains-clusters and proportion of low Sigma CSL boundaries decrease with the increasing strain, is caused by the increasing nucleation density of recrystallization with the increase of strain. The specimen with large size grains-cluster microstructure and high proportion of low Sigma CSL boundaries exhibits much better resistance to mass loss during intergranular corrosion testing than that with small size grains-cluster microstructure and relatively low proportion of low Sigma CSL boundaries.