A three-dimensional spray combustion simulation of a carbon black furnace is developed in order to predict the reaction behavior and yield of carbon black, The effect of the raw-fuel flow rate on the soot formation reaction is examined.The experimental furnace used in this study was one fifth the scale of a commercial plant. The light oil (LO) was fed as a fuel and the heavy oil (HO) as a raw-fuel. Both oils were derived from coal tar, Calculated gas temperature, pressure, gas species, and soot yields are compared with experimental data. The following results are obtained;1) An increase of the raw-fuel rate makes the gas temperature lower near the spray nozzles by its latent heat. The high temperature region caused by the heat of incomplete combustion shifts toward the downstream side of the furnace.2) Simulated results show the consumption of oxygen and formation of hydrogen and CO in a few milliseconds, and they are in good agreement with the experimental data.3) It is also found that both acetylene and methane are formed in the furnace. As the raw-fuel now rate increases, their maximum concentrations become higher and the positions of their maximum concentrations move toward the downstream side of the furnace.4) The soot yield increases as the raw-fuel flow rate increases. The calculated soot yields are in good agreement with the experimental data.