Phase equilibria and density of polymer in supercritical fluid are fundamental information for chemical process design and molecular structure analysis in polymer science. In this study, we measured high pressure phase equilibria and densities of poly(ethylene-co-butene)₁₀ and poly(ethylene-co-butene)₄₇ in two supercritical solvents; dimethyl ether(DME) and dimethyl ether-d(DME-d). A high-pressure variable-volume cell was used to measure the equilibrium properties. Linear variable differential transducer(LVDT) on the piston enables to measure the volume change of the equilibrium cell. High pressure phase equilibria of poly(ethylene-co-butene)₁₀-dimethyl ether were measured at 106-185℃ and pressure range of 486-1,100 bar. The similar system, polyethylene-dimethyl ether showed higher equilibrium line by 100-350 bar. The liquid-liquid equilibrium line of poly(ethylene-co-butene)₁₀-dimethyl ether and poly(ethylene-co-butene)₁₀-dimethyl ether-d existed within 110-150℃ below 1,000 bar. Phase behavior of poly(ethylene-co-butene)₁₀ and poly(ethylene-co-butene)₄₇ in dimethyl ether were also measured. Density of pure dimethyl ether was measured at 110℃, 120℃, 130℃, and 150℃ and pressure range of 139-2,415 bar. Mixture densities of poly(ethylene-co-butene)₄₇-dimethyl ether, poly(ethylene-co-butene)₁₀-dimethyl ether-d, and poly(ethylene-co-butene)₄₇-dimethyl ether-d were measured at the same temperature and pressure range.