Abstract: Compound specific carbon isotope analysis was conducted for crude oils collected from a drilling well and the entire oilfield at Lunnan, Tarim Basin, NW China, and the results demonstrated that there was an unusual distribution of δ¹³C showing that carbon isotope was enriched in Ordovician oils and depleted in Upper Triassic oils.This unusual distribution is consistent with the distribution and evolution of carbon isotope of crude oils from all over the world in geological history, and cannot be explained by simply utilizing conventional models based on the differences in organic facies or sources, maturity, depositional environment, or migration fractionation, etc..in consideration of the great span in geologic times-cale between Ordovician and Triassic periods.It is illustrated from a survey of geochemical literature that the unusual distribution of stable carbon isotope of crude oils was associated with the global paleoenviron-mental changes such as in atmospheric CO₂ and algae growth rates, etc., and was indeed resulted from the combination of the positive excursion of δ¹³C values in Ordovician period and the negative excursion in Triassic period.Crude oils, as a product of sedimentation and a kind of fossil fuels, recorded the paleoenvir-onmental and paleoclimatical changes in geological history, their stable carbon isotope is also controlled by these changes.