The solubilization capability of surfactants reaches its maximum when three-phase body consisting of water, oil and microemulsion phases appears. Considering the distribution of surfactants in the microemulsion phase, we analyze the three-phase behavior of mixed surfactant systems and obtain a theoretical equation to predict the three-phase equilibria. The equation is applied to polyoxyethylene-type nonionic surfactant mixtures. The difference in phase behavior of pure surfactant and commercial surfactants are analyzed according to our theory. The correlation between the three-phase (HLB) temperature and Griffin`s HLB number are also discussed.