Supercritical n-dodecane (C12H26) flows in heated circular pipes with thermal cracking were numerically investigated using a numerical preconditioning method for solving the compressible Navier–Stokes equations. The reaction rate constants for thermal cracking of n-dodecane were determined using Cantera. The thermophysical properties of pure n-dodecane and its decomposed components were calculated using the Helmholtz free-energy equation of state. The outlet temperatures agreed well with experimental results, and the maximum error was 3.2%. The outlet temperature increased with the wall temperature condition, although the rate of increase became slightly smaller when the wall temperature was high. We discussed the relationship between the wall temperature condition and heat transfer in terms of thermal diffusivity in the radial direction. We also compared the radial distributions with those of n-octane (C8H18) in terms of temperature, thermal diffusivity, and mass fraction of unreacted fed hydrocarbons. Thermal cracking mainly occurred in a high-temperature region near the heated wall. The density of the decomposed components was much lower than that of n-dodecane and n-octane, resulting in a significant decrease in the mixture’s density near the heating wall. The decomposed components affected the supercritical hydrocarbon flows owing to changes in their thermophysical properties. The thermal diffusivity due to the decomposed components and turbulence affects the temperature distributions and mass fraction in n-dodecane and n-octane flows. Finally, we compared the outlet conversion rates of n-dodecane and n-octane flows.