The nitrate removal rate of denitrifying granular sludge in an upflow sludge blanket reactor is very high and reaches up to 3.6 gNO3-N-gVSS-1 d-1 at a nitrate loading rate (NLR) of 32.0 gNO3-N L-1 d-1. However, the granular sludge exhibits flotation under high NLR conditions, where the granules become large in size (of approximately 3-5 mm), light, and easily adhere to gas bubbles. In order to decrease the flotation potentiality of granular sludge, three measures were taken as follows: reducing the size of the granules, increasing the density of the granules, and weakening the adhesion effect of sludge to bubbles. While, these measures did not completely eliminate the granular sludge flotation. The way to solve sludge flotation namely instability was to slow down the biomass growth rate. The main factors to reduce denitrifying bacteria growth rate were verified as relatively low operating temperature, carbon source with comparatively slow degradation rate, and low NLR. Therefore, controlling denitrifying biomass growth rate via reducing temperature, replacing methanol with glucose as carbon source, and decreasing NLR was able to improve the stability of the granular sludge.