The coexistence curves of (T-n) (T and n are temperature and refractive index, respectively) for the system of DMA+AOT+n-octane have been determined within about 7 K from the critical temperature (Tc) by measurements of refractive index at a constantmolar ratio of DMAto AOT (3.46). The coexistence curves of (T-n) have been converted to the coexistence curves of (T-φ) and (T-ψ) (φand ψare volume fraction and effective volume fraction, respectively) through the standard curves determined by measurements of refractive indexes at various volume fractions and temperatures. The critical exponent β was deduced from the coexistence curves, which was found to be inconsistent with the 3D-Ising value and approach the Fisher-normalization value 0.365 in a rather wide temperature range. The experimental results were also analyzed to obtain the critical amplitude B and the Wegner correction terms B1 to examine the diameters of coexistence curves, and to discuss the goodness of the order parameter.