Aerobic granules with mean sizes of 1300μm and 900μm were seeded with nitrifying activated sludge in sequencing batch reactors (SBR) in this study. The results show that ammonia removal efficiency of the small-size granules increased to 90% within 2 weeks, while big-size granules got to the same level in 3 weeks. It is speculated that small-size granules had greater increasing nitrification rate for their bigger surface area and greater absorption ability when seeded with nitrifying activated sludge, which caused fast seeding of nitrifying bacteria in R2. Through cycle analysis, it shows that it took about 2.5h for the both reactors to remove ammonia in the system, though the nitrification rate in R1 (0.033d-1 ) is lower that that of R2 (0.033d-1 ). The higher biomass concentration in R1 (10g/l) than that in R2 (6g/l) explains. At the same, it was found that total inorganic nitrogen (TIN) removal efficiency in R1 was higher than that in R2, i.e. 64.7% vs. 57.3%, which was resulted from the bigger anoxic zone in R1.