The phase behavior of an asymmetric ternary mixture composed of methane,n -butane and n-octane has been systematically studied in the near-critical region. The experimental work was performed by using a computer-controlled, dual-cell high-pressure PVT unit (made by ROP Corporation, France) equipped with video camera-monitor-recorder system and on-line Anton-Paar fluid density meter. The experimental measurements involve the compressibility factor of the compressed fluid, the bubble/dew point locus in the near-critical region, the mixture critical point, the liquid volume fraction in the two-phase region, and the composition-density data of the coexisting equilibrium phases. Such systematic data are scarce in the open literature, and are valuable for testing the performance of an equation of state/mixing rule in the near-critical region. The volume-translated Soave-Redlich-Kwong equation of state proposed by Peneloux and Rauzy (Fluid Phase Equilibria, 1982) was used to predict the experimental data and the results being evaluated.