In this work, the relationship between surface roughness of the membrane and membrane fouling was investigated with sodium alginate which is a kind of extracellular polymeric substances (EPS) and is regarded as a main foulant in membrane bioreactor (MBR). Cellulose acetate butyrate polymer which is superior in heat-resistance and mechanical strength was used as membrane material. The membranes having the gear-shaped structures of different heights on the outer surface were prepared by using the several spinnerets via thermally induced phase separation. The membrane having the higher projection on the outer surface showed higher antifouling property. The recovery of the permeability by backwashing also increased by the higher projection of the outer surface of the membrane. Although the membrane permeability decreased by adding calcium chloride into the alginate solution, the membrane with the higher projection also showed higher permeability and recovery by backwashing. The increase of the permeability and the recovery by backwashing was not owing to the increase of the outer surface area by the gear-shaped structure formation. From the observation of the membrane outer surface after filtration of sodium alginate and humic acid, it was revealed that foulants were accumulated in the valley between projections, while the top of the projection was clean. This is the reason for the higher antifouling property for the membrane with the higher projection structure. (C) 2010 Elsevier B.V. All rights reserved.