Separations by hollow fiber modules are fast compared with conventional extraction equipment because of the large surface area per volume. In these modules, the reactants can be contacted at high speed and two flows are completely independent, so there are no problems with loading and channeling. General the mass transfer coefficients in hollow fibers will be maximized when the resistance of the membrane is minimized. This membrane resistance will be minimized if the hollow fiber is wet by the solvent that flow the outside of the fiber In this paper, for the extraction of Co(II) from wastewater, the extractants were LA-2, EHPNA and TOPO. To determine the rate controlling step in hollow fibers, we examined the effect of inside and outside flow rates of the fibers. From these experiments, we identified for the extraction of system with partition coefficient in hydrophobic hollow fibers, mass transfer in the inside aqueous feed dominated the overall mass transfer.