This study investigated the feasibility of improving the stability of aerobic granules through selecting slow-growing nitrifying bacteria. For this purpose, four sequencing batch reactors were operated at different substrate N/COD ratios ranging from 5/100 to 30/100. Results showed that aerobic granules formed in all four reactors, and aerobic granulation was a gradual process evolving from the dispersed seed sludge to mature and stable granules, and the whole granulation process could be divided into three phases, i.e. acclimation phase, granulation followed by granule maturation. The observed growth rate and mean size of mature aerobic granules were found to decrease as the substrate N/COD ratio was increased, while nitrifying population was enriched markedly in aerobic granules developed at high substrate N/COD ratios. The enriched nitrifying population in aerobic granules was responsible for the observed low growth rate of aerobic granules. It seems certain that the substrate N/COD ratio is an important factor in selecting nitrifying bacteria in aerobic granules. Aerobic granules with low growth rates showed strong structure and good settleability in terms of specific gravity, SVI and cell hydrophobicity that further lead to high stability as compared to those having high growth rates. This study demonstrated that the selection of slow-growing nitrifying bacteria through controlling substrate N/COD ratio would be a useful strategy for improving the stability of aerobic granules.