To improve the antifouling property of polypropylene hollow fiber microporous membranes (PPHFMMs) in a membrane-bioreactor (MBR) for wastewater treatment, the PPHFMMs were subjected to surface modification by CO2 plasma treatment. Structural and morphological changes on the membrane surface were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Water contact angle, which reflects the hydrophilicity of the membrane surface, was measured by the sessile drop method. Results of XPS clearly indicated that the plasma treatment introduced oxygen containing polar groups on the membrane surface. The static water contact angle of the modified membrane reduced obviously at first and then kept almost constant with the increase of CO, plasma treatment time. To assess the relation between the plasma treatment and the membrane fouling in a MBR, filtration for activated sludge was carried out using synthetic wastewater. The PPHFMMs after CO2 plasma treatment showed better flux recovery after cleaning than that of the unmodified membrane. Fouling index (FI) for the CO2 plasma treated PPHFMMs was lower than that of the unmodified PPHFMM. © 2005 Elsevier B.V. All rights reserved.