Thermophysical properties of HFE-356mmz in the vapor phase were measured by means of an acoustic-microwave resonance method. HFE-356mmz, which is 1,1,1,3,3,3-hexafluoro-2-methoxypropane in chemical name, is expected to be used as a working fluid with low global warming potential for the Organic Rankine cycle (ORC). The sound velocity and dielectric permittivity were simultaneously measured by using a cylindrical acoustic-microwave resonator. The sound velocity data were analyzed to obtain the ideal-gas heat capacity at constant pressure. The integral of the ideal-gas heat capacity as a function of temperature derives the ideal-gas enthalpy, which is a fundamental and important energy property to simulate the thermodynamic cycle. Similarly, the analysis of the dielectric permittivity data leads to information on the ideal-gas molar polarizability, dipole moment, and density. The acquired thermophysical properties of HFE-356mmz were compared to those of R-245fa and n-pentane, which are the existing working fluids for the ORC system, to prospect a feasibility of HFE-356mmz as their alternative.