The effect of support material pore size on the filtration behaviors during start-up and stabilized stages in the dynamic membrane bioreactors (DMBR) was studied. Before the dynamic membrane (DM) was formed, the turbidity at 50-mu m could be more than 250 NTU, while it was less than 40 and 10 NTU at 25- and 10-mu m, respectively. After the DM was formed, the stabilized stage lasted for 61 days with low transmembrane pressure < 0.6 kPa and the 5-, 10-, and 25-mu m filters had similar effluent turbidity (< 1 NTU) and chemical oxygen demand. However, their averaged flux was 66.4, 25.1, and 3.5 L center dot m(-2)center dot h(-1), respectively, suggesting that the 25-mu m filter had significantly lower filtration resistance. Consequently, to avoid unallowable high effluent turbidity during start-up or after membrane cleaning and to achieve high flux with low pressure filtration, a mesh size of similar to 25 mu m is more suitable for DMBR.