Abstract: The correlation of carbon isotope of ethane and maturity is srudied by hydrous pyrolyses of carbonate rock, mudstone, oil shale, and coal. It is found that the carbon isotope of ethane both for sapropelic pyrolytic gas and humic pyrolytic gas obviousy shows an inheritance of mother materials, however, there is a differnece in their distributions for the two types of gas. In other words, the δ^l3C value of ethane for sapropelic natural gas is smal1er than -29%,(PDB), while the δ^l3C value for humic natural gas is greater than -29%,(PDB). It is concluded that the results either from hydrous pyrolytic experiments or from actual statistic figures indicate that δ^l3C value -29%, of ethane can be taken as a criterion to determine the genetic types of sapropelic or humic natural gas. It was used in studying the genetic types of natural gas in Tarim basin, which achieved good results.