A three dimensional spray combustion simulation of a carbon black furnace is developed in order to predict a reaction behavior and yield of carbon black. An experimental furnace used in this study is one fifth scale of a commercial plant where the light oil (LO) is fed as a fuel and the heavy oil (HO) as a raw material. Both oils are derived from coal tar. The carbon black is synthesized as follows; 1) spray combustion of LO using a hollow cone spray nozzle, and 2) spraying HO using four flat spray nozzles into the above-mentioned combustion gas in the downstream region. In order to obtain spray characteristics of LO and HO, spraying experiments using water are also conducted. The numerical calculation is carried out as follows; 1) calculating LO droplet trajectories by Lagrangian treatment and estimating the combustion flow field using the k-ε: turbulence model, and 2) calculating HO droplet trajectories and estimating the synthesis field of carbon black using the k-ε: turbulence model. The calculated temperature and the gas composition in the furnace are in fairly good agreement with experimental results. The obtained data revealed the soot formation behavior in the furnace accompanied by the disappearance of oxygen and formation of carbon monoxide, hydrogen and acetylene in a few milliseconds.