Soot plays an essential role in radiative heat transfer in pulverized coal boilers, but the controlling processes that cause different soot behaviors in O-2/N-2 and O-2/CO2 atmospheres are not clear. In this work, a 1-D transient model of the combustion of single coal particles with soot evolution was proposed and validated in both O-2/N-2 and O-2/CO2 atmospheres. Gasification reactions of tar and soot were included in the soot-relevant reaction pathways, and the reaction rates were compared quantitatively to reveal controlling processes affecting soot behaviors. The results show that the predicted soot volume fraction decreases apparently in O-2/CO2 environments when soot gasification and tar gasification are considered. The effect of tar gasification is more important than that of soot gasification. However, in O-2/N-2 environments, both soot gasification and tar gasification barely affect the predicted soot volume fraction. The decrease in soot volume fraction at the late stage of volatiles combustion is predominantly attributable to soot diffusion from the fuel-rich zone near the particle surface to the outer boundary rather than an actual decrease in soot yield. Among all soot-relevant reaction pathways proposed in the simulation, the controlling processes affecting soot yields in O-2/N-2 and O-2/CO2 atmospheres include soot formation, tar oxidization, and tar gasification. Finally, the effects of temperature and oxygen concentration on soot yields in O-2/N-2 and O-2/CO2 atmospheres are discussed.