The effects of different outer diameters and concentrations of multi-walled carbon nanotubes on polysulfone (PS) hollow fiber membranes were investigated. Nanocomposite polymer solutions were formed by adding 0.01, 0.05, 0.2 and 0.5 wt% multi-walled-carbon nanotubes (MWCNT), having 20-30, 8-15 and <8 nm outer diameters, to PS. Subsequently, fabricated membranes were characterized according to their dope solution viscosity; surface morphology, hydrophilicity and charge; membrane permeability. The effects of MWCNT addition on membrane performance were determined by electrochemical impedance spectroscopy (EIS) as well as by reversible and irreversible fouling ratios. EIS data showed that double-layer capacitance (C-dl) values of membranes were increased as negative charge density on membranes was increased. Moreover, EIS measurements indicated a decrease in the C (dl) dependent on the BSA filtration. This study demonstrated that EIS can be successfully used for hollow fiber membrane configuration. It also showed that EIS can be used for the surface characterization and membrane performance of hollow fiber nanocomposite membranes.