Filtration characteristics and mechanism of membrane fouling were investigated with MBR mixed liquor. It was found that, the variation of filtration resistance was fit to the standard blocking filtration model in initial process, and then fit to cake filtration model in later process. In dead-end filtration of sludge, the relative flux was in the tendency to the exponential decay along with filtration time, and flux was decreased more quickly at higher pressure. Sludge was compressed during filtration process, and resistance of sludge was increased with the increase in filtration pressure, the compressibility index was obtained as 0.807 8. Quantitative calculations showed that cake resistance made up over 90% of total resistance. In addition, the relative contribution of each of the sludge fractions to membrane was respectively calculated as 87.98 % for suspended solids, 6.20 % for colloids and 5.82% for solutes. Then a possible mechanism underlying membrane fouling was proposed.