The selective sludge discharge mode was studied in the aerobic granular sludge reactor for improving the long-term stability of aerobic granular sludge process. Results showed that the nitrogen removal performance was obviously improved with the preliminary granulation of the bioreactor at sludge settling time of 1 min after 15 days. After the formation of fine granular sludge, 5-10% volume of the total granular sludge was discharged for the sludge retention time (SRT) controlling, and the removal efficiencies of NH4+-N and TN reached 95.9 +/- 2.0% and 75.3 +/- 3.0% at the SRTTotal of 5.3 +/- 0.4 days, SRTGranules of 9.9 +/- 0.9 days and SRTFlocs of 2.7 +/- 0.5 days, respectively. The granular rate and average granular diameter of the bioreactor were stabilized at 80 +/- 5.0% and 2.3 +/- 0.3 mm, respectively. The results demonstrated that a long SRT of granular sludge leads easily into the deterioration of aerobic granule, and an appropriate selective sludge discharge mode favors the stability of aerobic granular sludge process via the discharge of proportional aging granular sludge and retention of enough new-born flocs. (C) 2012 Elsevier B.V. All rights reserved.