Abstract: The mixing of gases is ubiquitous in nature because of their special physical property. The mixed gas is formed by mixing of different genetic types of natural gases,and reflected obviously the gas geochemical features. If define one origin type of natural gas as one end member, the mixed gases would be divided into two. three and multiple end members. The common and significant type of mixed gas is of two end members which consists of three large groups: (a)mixing of gas with inorganic and organic origins ;(b)mixing of organic origin gases with different source type; (c)mixing of gases with different evloution stags; Gas migration, especially second migration that caused by diffusion and infiltration, is the main way which make the mixing of different types of gases. The first condition for mixing of natural gases is the existence of gas sources of different types and evolution stages. The chemical composition,stable isotopic composition of carbon and hydrogen and the noble gas isotopic composition are the main markers to differentiate mixed gases. Isotopic composition of the noble gas is an important tool to differentiate the mixed natural gases with crust and mantle sources,while the stable isotopes are to differentiate the mixed natural gases with different genetic types and evlution stages. It should be pointed out that it could effectively differentiate mixed gases by comobining geologic backgrounds and various geo-chemical parameters. By study the mechannism and conditions of mixed gases formation and analyse some examples,the mixing model of two -member gases was established.