Abstract: The highly mature light oil from well Shunnan 1 in the Guchengxu Uplift, Tarim Basin is considered to be a typical residual oil by thermal cracking. The study reveals that the abundant monoaromatic hydrocarbons and polynuclear aromatics in the crude oil of well Shunnan 1 are the by-products of severe in-reservoir oil-cracking during the geological history. The monoaromatic hydrocarbons are the intermediate products of thermal cracking, cyclization and aromatization, while the polynuclear aromatics are indicative of polycondensation reaction. The strong carbon isotopic fractionation induced by hydrocarbon molecule cracking and polycondensation makes the molecule carbon isotope of n-alkanes and the molecule carbon isotope of alkyl naphthalenes in the crude oil of well Shunnan 1 respectively 8‰-10‰ and 6‰-8‰ heavier compared to those from unaltered crude oils in Lower Paleozoic oil-bearing system. The results further proved that the aromatic molecules related to oil cracking and their stable carbon isotopic compositions could be a potentially powerful tool to identify whether thermal alteration occurred in deep and ultra-deep oil reservoirs, and further helps to effectively evaluate the oil and gas phase and resource pattern in the deep and ultra-deep reservoirs.