Synergistic extraction of NI(II) by a mixture of the active component in the commercial extractant LIX63, 5,8-dimethyl-7-hydroxydodecane-6-oxime, and di-oleylphosphoric acid (DOLPA) has been investigated by liquid surfactant membranes (LSMs) containing a commercial surfactant, Span 80. Solvent extraction of Ni(II) with the mixture of LIX63 and an ordinary extractant, di(2-ethylhexyl)-phosphoric acid (D2EHPA) was carried out. The results show that the LIX63-DOLPA mixture has a high synergistic effect on the extraction of Ni(II) in the LSM system compared with the LIX63-D2EHPA mixture. The synergistic effect of the LIX63-DOLPA mixture is attributed to a mixed ligand complex formation with the stoichiometry of Ni2+/LIX63/DOLPA = 1/2/2. Besides extractant synergism, the effects of several operation factors in the LSMs were investigated : permeation of Ni(II), pH of external aqueous phase, and concentrations of extractant and surfactant. The permeation mechanism of Ni(II) in LSMs has been elucidated by using the interfacial reaction model which takes into account the complex formation at the interface.