The hydrates in marine environment can be grouped into two categories,diffusion gas hydrates and vent gas hydrates. The diffusion gas hydrates occur widely in an area where bottom simulation reflector (BSR) was recorded in seismic profiles, and is a thermodynamic equilibrium system of hydrates and water with dissolved methane within gas hydrate stability zone (GHSZ). The hydrates are buried in a distance apart from the seafloor and are characterized by low concentrations. The vent gas hydrates occur in an area where gas vents out of the seafloor. It is a thermodynamic disequilibrium system of hydrate, water and free gas, occurs in a zone that extends from the base of GHSZ to the seafloor, and is characterized by high concentration. Reported evidences show that these two types of hydrates are possibly occurring in the South China Sea.