This paper is mainly concentrated on the geochemical characteristics and origin of gas of Kekeya field in the Tarim basin, NW China. Many dispute viewpoints  about origin of gas and condensate existed although the field has been found in 1977. This study shows that Permian mudstone is main source rock of oil and gas. The carbon isotopic value of natural gases in kekeya region are more heavy than that of other natural gases in Tarim basin which are mainly derived from lower  Palaeozoic marine source rocks. In addition, the 40Ar/36 Ar and 3 He/4 He ratios are also indirect evidence for gases mixing. The relationship between  carbon isotopes of methane and ethane is coincidence with Faber equation of gas derived from Organic matter Ⅰ, Ⅱ. This will also approve that gases are mainly  derived from source rocks which have relatively good organic matter type. Most maturity of gas is estimated as 1.8%~2.2% besides K₂ and K18 well based on the formula. They belong to overmature gases which are generated by Carboniferous-Permian source rocks. In addition, the gas derived from 0.9%~1.2%  Ro source rocks (Jurassic coal-bearing strata) may also be mixing.