In this paper, experimental investigation of wastewater treatment by ultrafiltration (UF) is introduced. In the experiments, a UF membrane (UFPHT20-6338) and samples of Behshahr Ind. Co. wastewater as feed were used. Effect of operating conditions such as pressure difference, cross flow velocity, shear stress, temperature, concentration of organic compounds and pH on permeation flux, flux decline and fouling resistance were studied. Results show that a pressure difference more than 3 x 10(5) Pa (3 bar), a high cross flow velocity (depending on economic considerations), a temperature of 30° C and a pH of 9 are the best operating conditions. Fouling of the membrane completely follows Hermia's model (cake filtration mechanism). Analysis of the wastewater treated by UF represents 91, 87, 100, 85 and 40% reduction in COD, TOC, TSS, [PO43-] and [Cl-], respectively. Also, treatment of the wastewater by UF-powdered activated carbon (PAC) was studied. According to results, using PAC in feed circulation loop of UF system with a concentration of about 0.1% (W) not only improves water quality but also increases permeation flux. A shear stress induced by PAC on the membrane surface decreases cake layer thickness and as a result increases permeation flux. Analysis of the wastewater treated by UF-PAC represents 94, 93, 100, 99 and 43% reduction in COD, TOC, TSS, [PO4-3] and [Cl-], respectively. Reduction of the phosphate concentration by UF-PAC is very considerable. A comparison between the results shows that UF is better than conventional biological method and UF-PAC is better than UF. © 2005 Elsevier B.V All rights reserved.