Supercritical octane flows that undergo thermal cracking in a heated circular tube were numerically investigated using our numerical methods based on the preconditioning method solving the compressible Navier–Stokes equations with the thermophyical properties from the Reference Fluid Thermodynamic and Transport Properties Database (REFPROP). The simple thermal-cracking model for octane was developed from the results calculated by KUCRS and Cantera software, assuming mild cracking condition and single-fluid flow. Supercritical octane flows were simulated under wall temperatures that would and would not encourage thermal cracking. Slight temperature differences affected the velocity distribution in the heated pipe because of the change in density. In low-wall-temperature conditions without thermal cracking, the simulated outlet temperatures were in good agreement with experimental results. Octane near the heated wall was decomposed by thermal cracking in the high wall temperature conditions. The wall temperature and the residual time affected the conversion rate of the thermal-cracking conversion rate. The present numerical methods underestimated the experimentally measured conversion rate, though the rough trends of the conversion rates were captured.