Powdered activated carbon(PAC) process was combined with crossflow ultrafiltration (PAC-UF process) to investigate the effect on permeability of ultrafiltration. Study was conducted with laboratory scale PAC-UF process and UF system. Secondary sewage effluent of RBC process was fed into two systems. PAC concentration of 450㎎/ℓ and contact time of two hours were maintained and the backwashing was carried out for 90 seconds with high pressure of 2.5㎏f/㎠ during operation. The following conclusions are drawn : 1) By adding PAC as a pretreatment process, the membrane fouling caused by refractory organic matters was controled. 2) When the duration of filtration was increased from 15 minutes to an hour, the permeate flux was significantly decreased because PAC layer accumulated on membrane surface and gel layer formed by adsorption of PAC. 3) In case of a dense membrane module, circulation of crossflow was hindered by accumulated PAC between housing and membrane. Thus, packing density of membrane module was to be designed in consideration of added PAC.