The feasibility of using flat-sheet membrane for waste activated sludge (WAS) thickening and digestion was studied in this paper. The characteristics of the processes including the thickening efficiency, effluent water quality and filtration properties of the membrane for sludge thickening process (MST), digestion efficiency, effluent water quality and membrane permeability of the membrane for sludge simultaneous thickening and digestion process (MSTD) were investigated. Test results showed that good sludge thickening efficiency and superior effluent water quality were obtained in the MST process under hydraulic retention time (HRT) 0.26 and 2 d operation of each cycle, and the membrane fouling was mainly due to the increase of apparent viscosity of mixed liquors and the decrease of the cross-flow velocity (CFV) along membrane surfaces during one thickening cycle. Membranes were also selected for the processes and M1 membrane of polyvinylidene fluoride (PVDF) material with pore size 0.2 microm demonstrated better permeability compared with other three membranes. About 80% MLSS destruction rate and 73% MLVSS destruction rate were achieved under HRT 1 d, dissolved oxygen (DO) concentration 0.5-1.5 mg/L, temperature 20-28 degrees C and 15 d operation for one cycle in MSTD process, and membrane fouling of MSTD process was attributed to not only the reason of MST process mentioned above, but also the change of sludge properties such as the increase of soluble COD and microbial polymeric substances including EPS, carbohydrate and nucleic acids, etc. of the mixed liquors.