Three-phase equilibrium conditions Lw-V-H (liquid water solution-vapor-hydrate) for methane hydrate in the presence of ethylene carbonate (C3H4O3) as additive in aqueous solutions have been determined experimentally by isochoric method. New experimental results are reported for pressures ranging from 4 to 22 MPa and for mass fraction of ethylene carbonate ranging from 0.05 to 0.24 (0.011–0.061 for mole fraction). The experimental results show that the addition of ethylene carbonate to the system shifts hydrate phase equilibrium to higher pressure and lower temperature. With the concentration of 0.240 mass fraction of ethylene carbonate additive in an aqueous system, the most significant decrease of the equilibrium temperature is about 3 K at a given pressure, in comparison to that of methane + water system without additive. The effect of ethylene carbonate on methane hydrate formation conditions was compared with that of cyclopentanone (C5H8O) and γ-butyrolactone (C4H6O2), which have similar molecular structures. The following phenomenon has been revealed: increase of the number of oxygen atoms in a molecule of this row shifts the hydrate phase boundary to inhibition region. Powder X-Ray diffraction (PXRD) results show that clathrate hydrate with sI structure formed in presence of ethylene carbonate within investigated range of concentration is pure phase of methane hydrate without inclusion of additive. It was also observed that there were some unknown peaks, which were not the same with peaks of pure ethylene carbonate. We speculate that they correspond to a solvate of ethylene carbonate with water.