In aqueous solution containing the NaCl of 0.25 mol/L, the micellization of binary mixtures of sodium dodecyl diamino sulfonate (C12AS) and octylphenol polyoxyethylene ether (10) (OP-10) have been investigated employing both the UV.vis spectroscopy method, using pyrene as a probe, and the tensiometric method. The values of critical micelle concentration (cmc) at different molar compositions in the mixture of C12AS/OP-10 were determined by both the UV-vis spectroscopy measurement and the tensiometric measurement. Within the framework of pseudophase separation model, some parameters (including the ideal values of mixed cmc, the interaction parameters, components and activity coefficients in mixed micelles, and thermodynamic parameters of micellization) were calculated using various models and treatments, e.g. the regular solution theory, the ideal model from the Lang model, the Rodenas’s treatment and the Motomura’s treatment, and the mechanism on effect of NaCl on the interacting behavior between two surfactants was discussed. The results show that as compared to the cases in the absence of NaCl, the addition of NaCl enhances non-ideal mixing and synergism between C12AS and OP-10 in all mixed systems, mainly resulting from the decrease of the steric effect of headgroups of two surfactants and the increase of the ionic strength in aqueous solution. Thermodynamic data show that on adding NaCl the micellization for the mixtures of C12AS and OP-10 can proceed spontaneously, and is an enthalpy-driven process owing to the variation of hydrophobic properties in aqueous solution.