The mechanism of carbon isotopic fractionation for gaseous hydrocarbons is revealed by investigating the residual liquid hydrocarbons in laboratory pyrolysates of n-octodecane. The results indicate that cracking and polymerization in the relatively low temperatures and dispropor-tionation reactions leading to light hydrocarbons and pol-yaromatic hydrocarbons at high temperatures are probably causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly observed in pyrolysis experiments. This study provides significant insight for quantitative modeling of natural gas δ13C values and aid in the identification and assessment of natural gases derived from oil cracking. The mechanism of carbon isotopic fractionation for gaseous hydrocarbons is revealed by investigating the residual liquid hydrocarbons in laboratory pyrolysates of n-octodecane. The results that that cracking and polymerization in the relatively low temperatures and disproporationization reactions leading to light hydrocarbons and pol-yaromatic hydrocarbons at high temperatures are probably causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly seen in pyrolysis experiments. This study provides significant insight for quantitative modeling of natural gas δ13C values ​​and aid in the identification and assessment of natural gases derived from oil cracking .