The three-phase behavior of a microemulsion was investigated near the tricritical point in a NaBr/water/hexadecyl-trimethylammonium bromide (CTAB)butanol/decane system at 25 degrees C. The three-phase body, consisting of water-rich, microemulsion (surfactant), and oil-rich phases, exists between two critical endpoints where the water (or oil) phase becomes identical with the microemulsion phase. It is found in the phase diagram that the three-phase body shrinks with the change of NaBr concentration in water, and eventually disappears at the optimum concentration 0.5 wt % of NaBr. Not only the salinity but also the butanol/decane ratio effects the reduction of the three-phase body. This system was characterized by means of density, interfacial tension, and static lightscattering measurements. The change of the density values of two phases near the tricritical point where the three-phase body is not observed, apparently showed that the oil droplets disperse in water (O/W type) in the lower phase, while in the upper phase the water droplets disperse in oil (W/O type). Under a fixed decane/butanol ratio, the tricritical point tends to be reached with increasing CTAB concentration. As the tricritical point was approached, at which two phases look bluish, the light scattering experiment showed that the microemulsion droplets in the upper and the lower phases increased in size and their sizes become of the same order. At the tricritical point, the microemulsions with the maximum solubilization of water or oil merge with each other due to efficient droplet size and strengthened interaction between the particles. Consequently, the bicontinuous structure was formed.