Rheological and physiological characteristics of sludge in a submerged membrane bioreactor (MBR) were studied in order to classify their effects on filtration properties and membrane fouling. Viscosity, mixed liquor suspended solids (MLSS), extracellular polymer substances (EPS), soluble chemical oxygen demand (SCOD), dewaterability in terms of capillary suction time (CST), proteins and carbohydrates in the mixed liquor and their correlations with membrane fouling were investigated during a long-term performance. Statistical analysis was. proteins and carbohydrates employed to specify the influence of temperature, viscosity, MLSS, EPS, CST, SCOD. in the mixed liquoron membrane fouling. It was found that high MLSS concentration with a high apparent viscosity not only changed rheological characteristics of the mixed liquor, but also affected oxygen transfer coefficient in the submerged MBR. Among all these variables, viscosity, MLSS, EPS, CST, SCOD, carbohydrates and proteins in the mixed liquor were found to have positive correlations with membrane fouling. CST had the greatest impact on membrane fouling, followed by SCOD, carbohydrates, EPS, proteins, apparent viscosity and MLSS, respectively. Test results demonstrated that temperature had negative correlations with membrane fouling rate.