This paper reports the thermodynamic inhibition effect of seven ionic liquids (ILs) on methane hydrate formation. The isochoric multistep heating dissociation pressure search method is applied for experimentally determining the phase boundary between hydrate-liquid-vapor (H-L-V) phases and liquid-vapor (L-V) phases in the pressure and temperature ranges of 3.45-13.28 MPa and 274.3-287.6 K, respectively. All the studied IL aqueous solutions are used at a mass fraction of 0.1. A comparison of the thermodynamic inhibitory performance of various ILs is carried out and reveals the predominant role of the type of anion of ILs. Considering the difficulty in directly measuring the dissociation enthalpies, the values of Delta H of methane hydrate are also calculated using the Clausius-Clapeyron equation. It is found that the mean dissociation enthalpies of methane hydrate in the presence of the seven ILs vary from 59.05 to 60.81 kJ.mol(1), and are very close to that in pure water.