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Methoxymethane (called dimethyl ether, DME) is recognized as a promising alternative refrigerant. Knowledge of the phase equilibrium behavior of DME-lubricant is essential for selecting a suitable oil. A long-term systematic research program on the phase equilibrium for systems of DME and oils was carried out in our laboratory, making the effort to determine its dependence on the molecular structure of oils, temperature, and pressure. In this article, phase equilibrium data of the DME + pentaerythritol tetraheptanoate system and the DME + pentaerythritol tetranonanoate system were reported from (293.15 to 353.15) K using the isochoric saturation method. Experimental data of phase equilibrium were modeled by the Peng–Robinson (PR) equation of state using the HVOS mixing rule and the NRTL excess-free-energy model. Moreover, the influence of the molecular weight of pentaerythritol esters on the phase equilibrium behavior of DME/ester mixtures was analyzed.