The membrane fouling behaviors and fouling mechanism of filamentous bulking sludge in membrane bioreactor (MBR) have been reported in recent studies. However, few studies have been conducted towards the causes and control of filamentous bulking sludge in MBR. In order to make clear the conditions of filamentous bacteria overgrowth in MBR, the impact of DO and organic loading rate on filamentous growth and its characteristics were investigated in this paper. The results showed that overgrowth of filamentous bacteria occurred under a relatively high substrate concentration (organic loading rate was 0.61 KgCOD/(KgMLSS•d), and DO was above 2 mg/L) in MBR. Nutrient removal effect can not be deteriorated under filamentous bulking sludge, with average 95, 57 and 92% removal efficiencies of COD, TN and TP respectively. The particle size of filamentous sludge was larger than that of normal sludge and the filamentous bacteria produced more SMP and EPS than floc forming bacteria.