Abstract: Carbon and hydrogen isotope composition of natural gas are effective geochemical measures to distinguish gas origin and type,provide insight of the correlation of different gas sources, and determine gas maturity. Study shows that, composition of carbon isotope of methane is mainly influenced by type of organic matters and thermal evolution. Composition of carbon isotope in heavy hydrocarbons, such as (ethane) and propane, is mainly determined by carbon isotope of organic matters of source rock. At the same time, it is apparently influenced by the degree of thermal evolution. Gas with abnormal carbon isotope composition has been found in the Liaohe Basin. This type of gas is distributed in the southern part of East Depression in the Liaohe Basin. The δ¹³C₁ value of carbon isotope of methane is -44‰ to -40‰, the (δ¹³C₂) value of ethane is -13‰ to -6.6‰, and the δ¹³C₃ value of propane is -6.1‰ to +3.3‰. Ethane and propane of this type of gas are abnormally rich in heavy carbon isotope. This is the first time that this special type of gas is found. Geochemical data and geological setting analysis have proved that,this type of gas should fall into the category of mixed gas.