Numerical simulation on combustion and pyrolysis reactions in a tubular reactor is carried out on a 100,000 t/a. naphtha cracking furnace. A complex arrangement of bottom burners is contained in the furnace model. The hydrodynamic and radiation models are included for calculating the flue gas flow pattern and heat transfer. A molecular reactions model is applied on the basis of a two-dimensional tubular reactor model. The results calculated indicate that there is recirculation of the flue gas at each side of the reactor tubes due to the high inlet velocity from the bottom burners, which contributes to the uniformity of flue gas temperature in the furnace. Higher temperature profiles of the tube skin are mainly located 15-20 m along the tubular reactor. The calculated pyrolysis product yield and the tube skin temperatures are in good agreement with experimental data.