As a fundamental study of gas hydrates, which are key substances in the energy and environmental problems, the thermodynamic stability for the pure and mixed gas hydrate systems is investigated. The intermolecular vibration energy between the host water molecules and the intramolecular vibration energies of guest species are measured by means of laser Raman spectroscopy. The structural phase transition controlled on the equilibrium composition of the mixed gas is observed in the (methane + ethylene) mixed gas hydrate systems, while each pure hydrate has no structural phase transition in the whole range in this study.