Kinetic modeling of stable carbon isotope ratios of methane is a useful method in simulating theaccumulative history of a gas poo1． The usage of modeling of stable carbon isotope ratios of other heavycomponents(C ～ C5)is rare because of their low content in natural gases． Comparison between ”Cz inconfined svstem and in open system showed that ”C2 is sensitive to the openness of a system． As ”Cz～5 ism uch m ore stable than ”C1 in migration and bacterial contam ination， ”C2～5 is as valuable as t 1 in assessingthe history of a gas accumulation． In this paper，coal from the Upper Paleozoic of the Ordos basin waspyrolyzed in a confined system and the kinetic modeling of 。C2 was developed． The modeled values of 8C1(一33．46‰ )and 2(一23．1O％。)from this coal were close to the data of natural gases produced in the UpperPa1eozoic，but different from those in the Low Paleozoic．A conclusion can be drawn that natural gases in theUpper Paleozoic are mainly derived from long in—situ cracking of coal，while gases in the Low Paleozoic werefrom the overmature marine organic matters of the Low Paleozoic．