This paper describes pilot test results obtained on cooling tower blowdown of a power plant in HeBei province, using both 0.1 mu m MF and 0.03 pm UF, which are outside-in hollow fiber membrane modules. In this pilot, the MF and UF modules were installed at the same system, although at different testing periods. Operation parameters such as flux, backwash (BW) time, BW frequency, frequency of air backwash (ABW) were changed; the MF and UF modules were operated at constant operating conditions during the same testing period. The study showed that both the MF and the UF membrane systems can produce permeate with high and consistent quality. The filtrate SDI,, of the permeate produced by both systems was below 3 for most of the time (except the period of flocculation addition). This water quality meets the requirements of the reverse osmosis (RO) system. In-line rapid pre-coagulation using polyaluminium chloride (PAC) at a concentration of 2.5-5 ppm did not decrease the filtrate silt density index (SDI,,) and may have even led to a sharp increase in filtrate SDI,,, especially when combined with polyacrylamide (PAM),. In addition, the injection of polymerized ferric sulfate (PFS) and sodium hypochlorite (NaClO) to the pretreatment system feed water had little influence on the filtrate SDI,,. Other water quality parameters such as total Fe, Cu, COD and colloidal Si were also measured. The UF membrane module reached a higher level of FPI than the MF membrane module did, while the latter had more stable performance than the former. The MF/UF test has achieved stable membrane permeability, and both MF and, UF modules were found suitable to provide adequate pretreatment prior to RO desalination.