A home-made static NMR cell with pressure up to 10 MPa was employed to observe the formation and dissociation processes of methane hydrate by in situ 1H and 13C NMR spectroscopies. Methane hydrate can be formed or decomposed in the temperature range of -5 to-13℃ at pressures between 4.0 and 7.0 MPa. The higher methane pressure, the formation or dissociation temperature of methane hydrate was higher. In situ 1H NMR experiments indicated that the critical size of the hydrate clusters is crucial for the formation of methane hydrate.