Abstract: The structure of nitrifying granules can remain stable for 305 days in a granule membrane bioreactor (GMBR) without hydraulic selection pressure; however, majority of the granule structure was actually inactive due to mass diffusion limitation. As a consequence, active biomass free of mass diffusion limitation only inhabited the top 60-80 µm layer of the nitrifying granules. There was a dynamic equilibrium between bioflocs and membrane, i.e., 25% of bioflocs attached on the membrane surface within the last nine days of the backwash cycle in synchronization with the emergence of a peak of soluble extracellular polymeric substances (sEPS) concentration around 47 mg L-1. Backwash can eventually detach and return these bioflocs to bulk solution. However, the rate of membrane fouling did not change with and without the biofloc attachment. In certain sense, the GMBR investigated in this study functioned in a similar fashion as integrated fixed-film activated sludge membrane bioreactor and thus defeated the original purpose of GMBR development. Mass diffusion problem and sEPS production should be key areas of focus in future GMBR research.