The investigation of viscosity control on phase transition in mixed surfactant system bas been carried out by inorganic salt, hydrotrope, surfactant composition and surfactant type. The viscosity increased as inorganic salt content increased in the low concentrated micellar phase. but, Hexagonal phase tranfered to Lamellar phase as inorganic salt content increased. Also, the viscosity decreased as inorganic salt content increased in the concentrated lamellar phase. The repulsion force of Inorganic salt in the solution in repulsive to the electrical double layer of the respective micelle. In this case, the viscosity increased or decreased in the respective phase as the directions of received repulsion force between the electrical double layer and inorganic salt. The efficiencies of hydrotropes were different in the respective phase. The mechanism and efficiencies of hydrotrope has been studied in the micellar phase and Lamellar phase.