Abstract: The carbon isotopic compositions of methane and ethane have been determined in 14 gas samples taken from the Pingluoba and Xinchang gas fields in the Western Sichuan Basin. The δ 13C 1 and C 2+/ΣC n values of the samples are higher than -3.92% and 0.8%,respectively. The results indicate that hydrocarbons of the both gas-fields entirely are generated from sedimentary organic matter during thermal cracking. The calculated hydrocarbon formation temperatures based on the fractionation values between δ 13C 2 and δ 13C 1 of the gas fields are 51～81 ℃ and 100～118 ℃, respectively. All δ 13C 1 and δ 13C 2 values of these gas fields display as a well function of depth, and results suggest that the hydrocarbons of Pingluoba and Xinchang gas fields have migrated vertically from deeply buried source rocks through sedimentary layers, and entered into overlying reservoirs.