The dissociation of methane hydrate at high pressure is studied by in situ Raman spectroscopy in a diamond-anvil cell. As for the Raman spectrum of sI methane hydrate, the v(1) band of CH(4) is split into two peaks v'(1) and v''(1), indicating the partitioning of CH(4) between the large (5(12)6(2)) and small (5(12)) cavities, respectively. With increasing temperature, the intensity ratio of Iv'(1)/Iv''(1) decreases obviously, and the d(Iv'(1)/Iv''(1))/dT is -0.079 K(-1). Additionally, the v(1) band of the dissolved CH(4) is close to v''(1) of the CH(4) molecule engaged in the small 5(12) cavity. This implies that, in the initial stage of hydrate formation, the abundance of small 5(12) cavities is greater than that of large 5(12)6(2) cavities.