A series of thermal simulation experiments were carried outwith crude samples（from the Lungu area ofTarim Basin）and samplesm ixed with elementary sulfur or nˉpropylmercaptan in a confined system of gold tubeˉautoclave.The influences of sulfur on cracked gas of oil and the isotope composition of hydrocarbon gas are studied through comparing the yields of experimental products and their isotope compositions in each experiment.The results show that adding sulfur or organic sulfide slightly inhibits，as awhole，the generation of methane and other hydrocarbon gas，but facilitates the production of nonˉhydrocarbon gases such as hydrogen and carbon dioxide.Besides，the amount of elementary sulfur or organic sulfide has an apparent effect over the composition of the products.Adding sulfurwill exert a significant impact upon the carbon isotope composition of hydrocarbon gas，causing the carbon isotope generated at low temperature to be heavier.The influence of organic sulfide over the products is especially significant，resulting in a positive difference of as high as7‰upon the carbon isotope ofmethane，and5‰upon that of ethane and propane.The study also suggests that the chem ical state of sulfur has something to do with its influence upon the composition of oilˉcracking products and that themechanism oforganic sulfide involving in cracking reaction is greatly different from thatof elemenˉtary sulfur.